Roger Federer continued his strong recent set victory over Jo-Wilfried Tsonga in the opening match of ending ATP World Tour Finals, while Rafael Nadal squeezed past Mardy Fish later on Sunday.

Federer, the defending champ and a five-time winner of the event, earned a 6-2, 2-6, 6-4 triumph on Sunday to start Group B play. Nadal was a 6-2, 3-6, 7-6 (7-3) winner against the American Fish in the other Group B match.

Only the top eight players qualified for the lucrative tournament at the O2 Arena in London.

Group A, which starts play Monday, features world No. 1 Novak Djokovic, British star Andy Murray, Spaniard David Ferrer and Czech Tomas Berdych.

The top two finishers from each round-robin group will advance to the semifinals.

Federer has been the hottest player of the eight, entering the tournament with a 12-match winning streak. He beat Tsonga for the Paris Masters crown last Sunday and also won in his native Basel the previous week, adding a couple of Davis Cup wins to his run of 10 straight ATP victories.

The seventh meeting of 2011 between Federer and Tsonga was dominated early by the Swiss superstar. Federer won the opening set in a mere 21 minutes, but the Frenchman returned the favor in a 30-minute second set.

A tight third set went on serve until the 10th game, when Federer won the first three points against Tsonga's serve. Tsonga managed to save one match point, but couldn't save the second to give Federer his 60th match win of 2011.

"In the first set, I didn't put one ball in the court, and that's it," said Tsonga. "Then I started playing better. I think he was a bit surprised, because I played so badly in the first set. It's better to start with a victory, but I'm still in the tournament and I will try to play my best tennis in the next match."

Federer improved to 7-3 lifetime against Tsonga, including 5-2 this year. The 16-time Grand Slam champ is tied with Pete Sampras and Ivan Lendl for the most titles at the season-ending event.

Nadal improved to 8-1 lifetime against Fish, but only by a slight margin. Fish fired seven aces and Nadal double-faulted five times. However, Nadal managed to pull out the three-hour victory by saving 8-of-11 break points.

Tsonga and Fish will square off Tuesday, while Nadal and Federer renew their rivalry. Nadal has won all three matchups between the stars this season, and most recently in the final at Roland Garros.

Monday's schedule opens with Murray against Ferrer, while Djokovic faces Berdych.