LONDON, Nov 7 —Rafael Nadal stayed unbeaten at the season-ending ATP World Tour Finals by defeating Stanislas Wawrinka in straight sets, ensuring that he’ll end the year as the top-ranked player in men’s tennis.

The Spaniard won 7-6 (7-5), 7-6 (8-6) at the O2 arena in London to become the first player at the eight-man event to qualify for the semi-finals.

Nadal, 27, who missed parts of the last few seasons because of injuries including a partially torn patella tendon and knee inflammation, needed two wins in London to keep the top ranking he took from Serbia’s Novak Djokovic last month.

It was “one of the best things that I did in my career: come back to the No. 1 after three seasons,” Nadal told reporters. “That’s very difficult in our sport, and after a very important injury. That’s an emotional thing for me. In the same time, to be able to qualify for the semi-finals is an extra thing.”

Top-seeded Nadal took a lead and served for the first set before the Swiss player broke back and forced the tiebreaker. Nadal broke to go up 3-1 in the second set, and Wawrinka got it back to 5-5 and pushed into another tiebreaker.

Wawrinka, seeded seventh, had never won a set against Nadal in 24 attempts coming into the match.

“I had the feeling that I couldn’t change the match, I couldn’t change the score,” the 28-year-old Wawrinka said. “I was playing the right tennis, but maybe did a few mistakes in the important points.”

Best performance

The last time Nadal played at the ATP Tour Finals, in 2011, he lost twice in the round-robin stage and failed to advance. His best performance at the tournament was in 2010, when he lost the final to record six-time winner Roger Federer.

Wawrinka beat Tomas Berdych of the Czech Republic in his opening match this week, while Nadal defeated fellow Spaniard David Ferrer, the No. 3 seed.

In yesterday’s second Group A singles match, fifth-seeded Berdych defeated Ferrer 6-4, 6-4 to keep alive his chances of qualifying for the semi-finals. The two players with the best results from each group advance.

Berdych next faces Nadal in two days, when Wawrinka meets Ferrer. In tomorrow’s Group B matches, tournament favourite Novak Djokovic of Serbia takes on Argentina’s Juan Martin del Potro and Federer of Switzerland plays Frenchman Richard Gasquet.

In the doubles competition today, David Marrero and Fernando Verdasco became the first team to qualify for the semifinals by beating Leander Paes and Radek Stepanek 6-4, 7-6 (7-5).

In the Group B doubles, Alexander Peya and Bruno Soares defeated Marcel Granollers and Marc Lopez 3-6, 6-4, 10-5. — Bloomberg