Ketola played his first
SM-sarja match for Karhut in 1964 at the age of 15 against
Ilves. The following year, 1965, Ketola won his first
Kanada-malja championship for the club. In the 1965–66 season, he made his breakthrough as the team's leading player. In the following season, he played his first season for the Porin Ässät, which is successor team of Karhut. Ässät finished fourth in the league. Ketola scored 25 points in his first season with Ässät. At the end of the 1960s, the German agent Mike Daski asked Ketola about his interest in joining the
Detroit Red Wings in the
National Hockey League. Ketola agreed to go to the training camp, even though he had just signed a contract with the
Jokerit Helsinki. Ketola didn't make the cut to the Red Wings lineup. However, Ketola was offered a contract with the farm team
Grand Rapids Griffins, but due to the low salary, Ketola was not interested. He decided to return to Finland with Jokerit, with whom he still had a contract. With the Jokerit, Ketola set the club's one-game points record with eight points against Hilpara in January 1970. For the following season, 1970–71, Ketola moved back to Ässät, who won the Kanada-malja championship for the first time. Ketola got a lot of playing time and was one of the team's leading players with 31 points, which won him the top scorer award. In the fall of 1974, together with Heikki Riihiranta, Ketola made history by becoming the first player who played junior ice hockey in Finland to become a professional in North America, for the Winnipeg Jets of the World Hockey Association. Being big and not afraid of a physical game, he most often played as the team's second center, but sometimes also as the first center, with
Bobby Hull and
Ulf Nilsson playing as wingers. In 1976, the Jets won the WHA championship, the Avco Cup. In the middle of the next season, Ketola was sold to the Calgary Cowboys. However, the club soon went bankrupt, so Ketola returned to Finland and Ässät. In 1978, as captain of the team, Ketola won his third Kanada-malja championship, when Ässät beat
Tappara in the finals. In the decisive final match, Ketola scored six points, as Ässät beat tappara 6–2. His 20 points in ten playoff games was a long-time
SM-liiga record. In 1980, Ässät lost to
Helsingfors IFK in the finals. The decisive game ended with a 6–5 victory for HIFK, even though Ässät had equalised a 1–5 loss position. After the match, Ketola threw his silver medal into the stands of the
Helsinki Ice Hall. Ketola played his last SM-liiga matches in 1981. Having already retired from ice hockey, Ketola reverted the decision to retire to play in North America in the NHL with the
Colorado Rockies for the 1981–82 season. However, he got little playing time. In 44 matches, he scored 9 goals and 5 assists. Ketola played the last six matches of his playing career in
I-divisioona, Finland's second division, with
KalPa in the 1982–83 season. The team was coached by his friend
Matti Keinonen. == International play ==