Rafael Nadal of Spain serves to Mardy Fish of the U.S. during their singles tennis match at the ATP World Tour Finals in London Nov. 20, 2011. Nadal won 2-1. (Xinhua/Tang Shi)

LONDON. Nov. 20 (Xinhua) -- World No. 2 Rafael Nadal fought for almost three hours to overcome debutant Mardy Fish at the ATP World Tour Finals on Sunday.

Nadal met Fish, his first opponent in this year`s Finals after Swiss legend Roger Federer beat Frenchman Jo-Wilfried Tsonga 6-2, 2-6 and 6-4 in the opening match in the afternoon.

The Spaniard took the first set 6-2 but lost the second to American top seed Fish 3-6 before fighting until the tiebreak to grab the win 7-6 (3) in the final set.

"It was a very important victory for me, for my confidence, to start the tournament with positive feelings," said Nadal who revealed that his stomach got something wrong during the match and that`s why he went to the toilet after leading 2-0 in the third set.

"The third set was a little bit more crazy, up and down. But I felt very bad from the stomach since the beginning of the third. I was seriously really lucky for the victory, and I for sure am very happy," said the last year`s runnerup.

Nadal will face defending champion Federer in his next group match, whom he believed was in good shape.

"It will be a challenge for me. He`s playing fantastic, winning two tournaments in a row, and now today. It will be a very difficult match. I will try."