Spain's Rafael Nadal plays a return to Mardy Fish of the U.S. during a round robin single tennis match at the?ATP World Tour Finals at o2 Arena in London on?Sunday, Nov. 20, 2011.

Second seed and last year's finalist Rafael Nadal defeated eighth seed Mardy Fish 6-2, 3-6, 7-6(3) on Sunday in Group B at the ATP World Tour Finals.

In a tense evening match at London's O2 Arena, Fish recovered from a nervous opening set in his debut at the tournament and saved two match points at 5-4 in the third set. Despite struggling with illness, Nadal was able to clinch it in the tiebreaker.

Nadal sealed the opening set in 34 minutes in front of a 17,500 capacity crowd watching on in the dimly lit arena but, just like Federer earlier, he was rocked by an opponent playing some electrifying tennis.

Fish, who cracked the world's top 10 this year, let rip with some punishing forehands and displayed some exquisite touches at the net as he outplayed the 10-times major winner in the second set.

Momentum shifted immediately at the start of the deciding set, when Nadal took a 2-0 lead. However, Fish won three straight games but lost the sixth game to love for 3-3. As the match drew closer to tie-break territory, Nadal stepped up. At 4-5, Fish twice saved match points with gutsy runs to the net and kept his nerve to hold serve.

Nadal improved to 19-8 in tie-breaks this year, by raising his game to win four of the first five points. The third set alone lasted 84 minutes.

"I felt very bad from the stomach since the beginning of the third (set)," Nadal said. "I was seriously really lucky for the victory, and I for sure am very happy."

Nadal has picked up three titles this year, but has not tasted silverware since lifting his sixth trophy at Roland Garros in June. He has a 67-13 match record on the year.