London: Six-time champion Roger Federer beat world No.l 8 Jack Sock in the opening singles match of the ATP World Tour Finals at the O2 Arena here.
Second-seed Federer, 36, needed one hour and 31 minutes to defeat the American debutant 6-4, 7-6 (4) on Sunday.
In the day's other match, German 20-year-old Alexander Zverev, the third seed, fought back from 3-1 down in the final set to beat fifth seed Marin Cilic of Croatia 6-4, 3-6, 6-4.
Federer broke Sock in the first game as Sock, winner of the Paris Masters seven days ago, grew in confidence by using his powerful forehand to pierce holes in Federer's game, but he was unable to break back.
The 19-time Grand Slam winner wasted five break points in the second set before Sock forced the set into the tie-break, where Federer took a 5-4 lead and then served out for victory.
"I'm happy that I was able to come out today and had actually good energy," Federer said. "This is the best I've felt since the Del Potro final (in Basel). I'm very happy to see that I didn't have to pay the price for taking it easy and resting and recovering.
"I think we're all going to start playing better every round that goes by," Federer added.
Federer is now 53-12 at the ATP Finals, which he first qualified for in 2002 when the tournament was played in Shanghai.
After the match, Rafael Nadal was honoured on court, receiving his ATP World Tour No. 1 trophy for finishing as the year-end No. 1 in the world rankings.
"It's been a fantastic season, a very emotional one after all the things I've been going through the last couple of years," said Nadal. "It's not something I could've thought of one year ago, so it means a lot to me."