Early career Zieler was born in
Cologne, North Rhine-Westphalia. He began his football career with his hometown club SCB Viktoria Köln before joining
1. FC Köln in 2001. In July 2005, he moved to England as a trainee with
Manchester United. By the time he moved to Manchester United, Zieler finished his education in Germany.
Manchester United In his first season with Manchester United, Zieler played 22 times for the Under-18 side, including four in the FA Youth Cup. He was also named as an unused substitute in 11 reserve matches, including the final of the
Manchester Senior Cup, earning him a winners medal. The following season, he shared Under-18 goalkeeping duties with
Ben Amos, but also made his debut for the reserves on 15 March 2007, a 3–0 home defeat to
Sheffield United. Despite that inauspicious start, he made amends four days later by keeping a clean sheet in the 2–0 Manchester Senior Cup win over
Oldham Athletic. The 2007–08 season marked Zieler's emergence as a full member of the reserve team, as he and
Tom Heaton shared the majority of the goalkeeping duties, each making 11 appearances. In May 2008, Zieler won another Manchester Senior Cup winners medal, as he played the full 90 minutes and kept a clean sheet against
Bolton Wanderers. He then played in the final of the Lancashire Senior Cup at the end of July 2008, restricting
Liverpool to just two goals as United won 3–2. At the start of the 2008–09 season, Zieler was given a first team squad number – 38 – for the first time. He got his first taste of first team action on 23 September 2008, when he was named on the bench for United's 3–1
League Cup win over
Middlesbrough.
Northampton Town (loan) On 28 November 2008, Zieler was sent on loan to
Northampton Town until 31 December as cover for their one senior goalkeeper,
Chris Dunn, as
Frank Fielding's loan from
Blackburn Rovers was coming to an end. The loan was subsequently extended until 31 January 2009, and then extended again on 2 February until 25 February. Zieler made his debut for Northampton Town on 21 February 2009, playing for the full 90 minutes of the Cobblers' 2–0 home defeat by
Walsall. He then played again three days later, in a 1–1 draw away to
Brighton & Hove Albion, before returning to Manchester United on 26 February after his loan spell reached 93 days. The loan could not be extended as 93 days is the maximum loan spell allowed outside the transfer window.
Back to Manchester United Upon returning to Manchester United, Zieler recommenced his duties as reserve team goalkeeper, conceding just one goal in his first four games back. However, in a match against
Newcastle United Reserves on 30 March 2009, he suffered a broken arm after colliding with a Newcastle striker in mid-air. He returned to action in the 2009–10 pre-season, but struggled to regain his position in the reserve team; his appearances throughout the season were sporadic and he was most often named as an unused substitute.
Hannover 96 On 22 April 2010, following a trial period, Zieler signed a two-year contract with German side
Hannover 96. The contract began on 1 July 2010 and also included the option for a one-year extension. Upon joining the club, Zieler was given a number 20 shirt. His consistent performances for Hannover 96 II earned him a promotion to the first team, appearing on the substitute bench against
Borussia Mönchengladbach on 4 December 2010 and made his Bundesliga debut on 16 January 2011 away at
Eintracht Frankfurt; he kept a clean sheet as his team won 3–0. Since his debut, he was made the first-choice goalkeeper for the rest of the season, dropping
Florian Fromlowitz to the bench. However, Zieler suffered a shoulder injury and was substituted as a result after playing 52 minutes, which saw Hannover 96 lose 4–0 against his former club,
1. FC Köln, on 11 March 2011. After returning to training, Zieler returned to the first team, in a 1–1 draw against
Mainz on 9 April 2011. Zieler went on to make fifteen appearances for the club in his first season. Ahead of the 2011–12 season, it was announced on 24 June 2011 that Zieler signed a contract extension, keeping him until June 2015. It came after Zieler was linked with a move to
Schalke 04 as a replacement for
Manuel Neuer, who joined
Bayern Munich. He also switched to the number 1 shirt previously worn by Fromlowitz, who transferred to
MSV Duisburg. The 2011–12 season saw Zieler make his first Europa League appearance in the Qualification Round against
Sevilla, which saw them win 3–1 in the first leg. In the return leg, Zieler conceded one goal, but helped the club draw 1–1 and helped them go through to the Group stage. Zieler remained the club's first choice goalkeeper for the 2011–12 season, both in the league and in the cup competitions, playing 4410 minutes in all competitions. In the 2012–13 season, Zieler started the season well when he saved two penalties in the second round of DFB-Pokal in the penalty shootout against
Dynamo Dresden; Hannover won 5–4 after playing 120 minutes and went through to the next round. In both encounters against Bayern Munich in the league, Zieler conceded eleven goals against the club, as Hannover lost 5–0 and 6–1 on 24 November 2012 and 20 April 2013 respectively. Despite this, Zieler continued to remain the first choice goalkeeper for the 2012–13 season, both in the league and in the cup competitions, playing 4440 minutes in all competitions. Ahead of the 2013–14 season, Zieler was linked with a return move to England, with
Liverpool keen on signing him, but was ruled out of a move, due to Hannover keen on continuing developing Zieler. Zieler remained at the club throughout the season and started well when he kept a clean sheet in the opening game of the season, with a 2–0 win over
Wolfsburg. Zieler then made his 100th league appearance for the club on 21 December 2013 against
Freiburg, but was booked in the 88th minute after fouling
Admir Mehmedi, resulting in him being substituted and Hannover losing 2–1. Despite this, Zieler continued to remain the first choice goalkeeper for the 2013–14 season, both in the league and in the cup competitions, playing 3240 minutes in all competitions. Ahead of the 2014–15 season, the club was keen on extending Zieler's contract, with his contract expected to expire at the end of the season. However, the talks over a new contract for Zieler were set to start after the conclusion of the World Cup. On 9 August 2014, Zieler signed a contract with the club, keeping him until 2017. Zieler made his 120th appearance, breaking Horst Podlasly's record of 120 consecutive appearances, in a 4–3 loss against
Hoffenheim on 29 November 2014. After conceding fourteen goals in four games between 22 November 2014 and 13 December 2014, Zieler ended the drought when he finally kept a clean sheet on 16 December 2014, in a 2–0 win over
Augsburg, ending a four match losing streak. Zieler went on to remain the first choice goalkeeper for the 2014–15 season, both in the league and in the cup competitions, playing 3240 minutes in all competitions. Ahead of the 2015–16 season, newly appointed manager
Michael Frontzeck expected Zieler to be the new captain following the departure of
Lars Stindl. Instead,
Christian Schulz was appointed as the club's captain. Zieler captained his first match at Hannover 96 on 12 September 2015 against
Borussia Dortmund, which saw Hannover 96 lose 4–2. In a 2–1 win over
Hamburg on 1 November 2015, Zieler broke the club's record of 153 appearances; a record formerly held by Hans-Josef Hellingrath. Zieler went to captain Hannover on ten more occasions in the 2015–16 season, but was unable to help the club avoid relegation. Despite the relegation, Zieler continued to remain the first choice goalkeeper for the 2015–16 season, both in the league and in the cup competitions, playing 3240 minutes in all competitions. It was announced on 16 April 2016 that Zieler would leave the club following the club's relegation, despite having a year to his contract left, citing his desire to continue playing top–flight football. Zieler was linked with a move to
Manchester City,
RB Leipzig and Köln. With his 3.5 million Euro release revealed, Zieler's departure to Hannover appeared to be increasingly imminent. During his time at Hannover 96, Zieler was ever-present in each of Hannover's 34 league games over the next five seasons, which culminated in their relegation. He faced more shots on target than any other goalkeeper in the
2015–16 Bundesliga season, making a total of 148 saves, second behind
Oliver Baumann, with 90 of them coming from inside the
penalty area.
Leicester City On 3 June 2016, Zieler returned to England for the first time in six years, signing for champions
Leicester City on a four-year deal for an undisclosed fee. Upon joining the club, Zieler stated that the chance to join the English Champions and play in the Champions League was too good of an opportunity to turn down and was allocated the number 21 shirt. Zieler made his Leicester debut on 27 August 2016, replacing the injured
Kasper Schmeichel in the 57th minute of the club's 2–1 home win against
Swansea City. However, his spell at the club was not to be a successful one: having failed to dislodge Schmeichel from the starting line-up, Zieler made only 13 appearances for Leicester. Given that the team had another reserve goalkeeper in
Ben Hamer, the club chose to sell Zieler to
VfB Stuttgart for an undisclosed fee after just one season.
VfB Stuttgart On 11 July 2017, Zieler signed a three-year contract with newly promoted Bundesliga side
VfB Stuttgart. In his
first season at the club, the team performed well, ending the season in 7th place, while Zieler started every match. During his
second season, Stuttgart experienced a severe slump, which resulted in a 16th-place finish and the club's eventual relegation, after losing to
Union Berlin in the 2018–19
play-offs. Although Zieler again started in every match, the goalkeeper had a difficult season. In a match against
Werder Bremen on 29 September 2018, he was involved in a memorable incident: after a throw-in from his teammate
Borna Sosa, he caused an own goal by briefly touching the ball as it rolled towards goal. The own goal generated significant media attention since the goal would not have counted had it not been touched by Zieler before crossing the goal line. Stuttgart went on to win the game.
Return to Hannover 96 After being relegated from the Bundesliga, Stuttgart elected to re-organise its goalkeeping position and to move on from Zieler. Thus, it was announced in June 2019 that he would return to his long-time club Hannover 96, who had also been relegated that season. The club played an underwhelming
2019–20 season in the
2. Bundesliga and failed to return to the Bundesliga after a sixth-place finish. Zieler played as the team's starting goalkeeper for most of the year but fell out of favour with manager
Kenan Koçak. Therefore, at the beginning of the
2020–21 season, the club announced that he was no longer in its first-team plans. In early August 2020, club chairman
Martin Kind stirred controversy by publicly belittling Zieler and encouraging the player to seek a new employer.
1. FC Köln After Hannover 96 made it clear that Zieler would no longer be part of their playing squad, it emerged that his former youth club
1. FC Köln were interested in his services. On 13 August, the club announced they had signed Zieler on a one-year loan deal. Zieler returned to 1. FC Köln permanently ahead of the
2025–26 season on a two-year contract. ==International career==