Early career Petsos started his football career at the age of eight for Düsseldorf 99 SC. In 2001, he was lured away to professional club
Bayer Leverkusen. He progressed through the youth system and was part of the team that finished runner-up in the
Under 19 Bundesliga in 2010. He made his debut for the Leverkusen reserve team in March 2010. He made seven more appearances in the 2009–10 season while continuing to play for the under-19 team. He made his
Bundesliga debut on 24 April 2010 against
Hannover 96. With a Leverkusen contract running until 2014, Petsos was loaned out to
1. FC Kaiserslautern for one season in 2010. He made his full
Bundesliga debut for the club on 13 November 2010 against
VfB Stuttgart being played as a right back. In April 2011, it was announced his loan to Kaiserslautern would be extended for the following 2011–2012 season. He made his debut for the club on 18 August 2012 in a
DFB-Pokal match against
Kickers Offenbach, he then went on to make his
Bundesliga debut on 31 August 2012 against
Mainz 05. He went on to make a total of 15 appearances for the club.
Rapid Wien On 15 July 2013, Petsos joined
Austrian Bundesliga club
Rapid Wien on a three-year agreement. He made his debut for the club on 27 July 2013 against
Wiener Neustadt. On 3 May 2015, in a 2–2 draw away to
SK Sturm Graz he scored for 2–1 with a hard long-range shot. On 4 August 2015, he greatly contributed to 3–2 away win with a strong performance in the third qualifying round of the
2015–16 UEFA Champions League against
Ajax. In early September, he suffered an injury during training with the Greece national team keeping him out of action for several weeks. Rapid in October stated their intention to extend Petsos' contract after he had been one of the team's best players during its strong start to the season. On 18 October 2015, he returned to the team, starting in an
Austrian championship game against
Wolfsberger AC, but suffered an injury in the first quarter of the match. On 22 October 2015, in a
UEFA Europa League 3–2 home game against
Plzeň, he scored a long-range goal, giving his club a lead of two goals. In January 2016, he announced he would not accept Rapid's offer to renew his contract which was set to expire in summer 2016 and would instead leave the Austrian club after three years. He left the club having 101 appearances (7 goals, 6 assists) in all competitions.
Werder Bremen A few days later Petsos signed a contract until summer 2019 with German club
Werder Bremen. On 11 September 2016, he made his debut with the club as a substitute in a 2–1 home loss against
Augsburg. On 2 January 2017, besides his new contract,
Werder Bremen announced that Petsos and
Janek Sternberg would be able to train in the U23.
"With the commitment of Thomas Delaney and the return of Florian Grillitsch, the competition for Thanos has become even bigger," said Managing Director
Frank Baumann.
"And Janek had a very limited amount of play in the preliminary round, and his situation is unlikely to change in the previous round, so they are now on the right track at U-23." Petsos fell out of favor with
Alexander Nouri, not least because of his critical comments about the coach. Nouri had announced that he would like to take up a maximum of 25 field players to work intensively. Having returned from his loan to Rapid Wien in summer 2018, Petsos spent the last year of his contract outside of the squad. Three days later, he made his Fulham debut as a substitute in an
FA Cup tie against
Hull City.
Loan to Rapid Wien On 30 August 2017, Petsos returned to former club
Rapid Wien, joining on loan for the 2017–18 season. Rapid Wien secured an option to sign him permanently from Werder Bremen.
WSG Swarovski Tirol In January 2020, after being out of contract for half a year, Petsos joined
WSG Swarovski Tirol, last in the
Austrian Bundesliga table.
Riga In February 2022, despite his contract with Tirol, Petsos will continue his career in
Riga, for an undisclosed fee. ==International career==