World No 1 Novak Djokovic began his bid for a record-equalling sixth ATP Finals title with a straight-sets win over Casper Ruud on Monday.
Djokovic, Finals champion in 2008 and every year from 2012 to 2015, dropped just four points on his first serve to seal a 7-6, 6-2 victory against the Norwegian debutant in Turin.
The 34-year-old Serb is looking to cap another remarkable year by joining Roger Federer as the season-ending tournament's most successful player, and he got off to a strong start against a player who appeared unfazed in his first ATP Finals match.
Ruud, the world No 8, could easily have taken a one-set lead. The 22-year-old came out firing by breaking Djokovic in the very first game, but by game seven the 20-time Grand Slam champion was in front 4-3 and looked well placed to take the first set.
Djokovic arrived at game 10 with his one-game lead intact and the match apparently turning in his favour but Ruud saved two break points to hold his serve and eventually take the set to a tie-break.
Ruud held a 4-3 advantage but Djokovic turned up the pressure by winning four straight points to claim the first set, and from there he raced clear.
Djokovic immediately broke Ruud at the start of the second set and he ultimately strolled to victory, breaking one more time to claim the second set and the match in 90 minutes.
It was Djokovic's sixth straight match victory having claimed the Paris Masters title earlier this month. That triumph in the French capital came in his first tournament since losing the US Open final, which scuppered his chances of achieving the calendar Grand Slam following titles at the Australian Open, Roland Garros, and Wimbledon.
Djokovic, who will end the year as world No 1 for a record seventh time and was presented with the trophy following his win over Ruud, will also face Greek world No 4 Stefanos Tsitsipas and fifth-ranked Andrey Rublev from Russia in the ATP Finals' Green Group.
The top two after the round robin stage will advance to the semi-finals, where the group winner will face the runner-up in the Red Group, whose winner will face the Green Group's runner-up.
The Red Group features Russia's world No 2 Daniil Medvedev, who beat Djokovic in the US Open final for his first major title, German world No 3 Alexander Zverev, seventh-ranked Matteo Berrettini from Italy, and Polish world No 9 Hubert Hurkacz.