London: World No.2 Rafael Nadal fought hard for three hours to overcome debutant Mardy Fish at the ATP World Tour Finals here.

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, 6-4 in the opening match Sunday 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,' he said.

'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 runner-up.

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.'

Meanwhile, Federer, 30, dominated the first set by breaking twice, but made more errors in the second set, giving Tsonga chances to make revenge. In the final set, the defending champion waited until the tenth game to break, grabbing the first win at the O2 Arena this year.

'I thought I wasn't playing that poorly,' said the 16-time Grand Slam champion Federer. 'You just try to stay calm. Absolutely it's not always in your control when you play Jo. That's the beauty of it. I kind of enjoy that to some degree.'