Norway's third seed Casper Ruud downed ATP Finals debutant Felix Auger-Aliassime 7-6(4), 6-4 on Sunday to kick-start his round-robin campaign in the season-ending tournament.
Canadian Auger-Aliassime has been in sublime form at the end of the season, winning three tournaments in a row before a semi-final run at the Paris Masters, but was not at his best in the opening match in Turin.
Although he fired 26 winners past world number four Ruud, they were offset by 26 unforced errors as the Norwegian wrapped up the victory to get on the board in the Green Group, which also has top seed Rafael Nadal and American Taylor Fritz.
"I think this is some of the best level that I've played since the U.S. Open," said Ruud, who reached the final of the year's last Grand Slam in New York.
"The last couple of months have been a little bit of a struggle, I have to honestly say that, but you have to accept it as well. You will face difficult moments in your career, and maybe these last couple of months was one of those."
Auger-Aliassime had won 16 matches in a row to win titles in Florence, Antwerp and Basel last month.
But Ruud, a semi-finalist last year, stamped his authority in the tiebreak and did not face a single break point in the match as he beat the Canadian for the second time this year.
"Felix has been on a roll this fall and I knew he was going to come out strong," Ruud added.
"I knew if I wanted to have any chance against any of the guys in my group I would have to bring my A-game and today I was able to do so."
Nadal plays Fritz later on Sunday.
