LONDON - Rafa Nadal opened his account at the ATP World Tour Finals with a 6-2 3-6 7-6 defeat of American Mardy Fish in a late-night Group B thriller at the O2 on Sunday.

The world number two was stretched to the limit by Fish who produced some dazzling tennis in his first match at the season-ender having cracked the world's top 10 this year.

Nadal took advantage of a slow start by his opponent to dominate the first set but the 29-year-old Fish responded by winning the second on his fifth set point with one of many sumptuous volleys he produced in the near three-hour duel.

Nadal, who had not played a competitive match for a month leading into the Finals, moved 2-0 ahead in the decider but after he rushed off court for a toilet break that Fish reeled off the next three games when he returned.

The Spaniard broke back and had two match points at 5-6 on the Fish serve, failing to convert either chance.

Perspiring heavily on the quick indoor surface, Nadal moved 4-1 ahead in the tiebreak and when three more match points arrived he made no mistake, sealing victory when the leaping Fish netted a high volley.

Federer off to winning start

Roger Federer narrowly avoided another Jo-Wilfried Tsonga ambush in London on Sunday as he began the defence of his ATP World Tour Finals title with a 6-2 2-6 6-4 victory as round-robin action began at the season-ending showpiece.

The Swiss endured one of the most painful losses of his year at Wimbledon in June when Tsonga blasted back from two sets down in their quarter-final and a repeat looked a distinct possibility in front of a 17,500 full house at the O2 Arena.

Federer, bidding for a record sixth title at the tournament, looked on course for a quick-fire victory when he ripped through the opening set in 21 minutes but he lost his way early in the second and the outcome was in the balance until Tsonga wavered in the final game of an entertaining opener.

"It's not always in your control when you play Jo," world number four Federer, who had former France soccer international Thierry Henry in his support box, told reporters. "Today I had flashes of that (Wimbledon) match because I didn't have much chance for a while on his serve.

"Once he got the upper hand in the second set he started to swing more freely and got really dangerous. I just tried to stay calm and wait for my chance.

"We saw some ups and downs from both players, that's why I'm pretty happy to come through."

Despite ending the year without a grand slam title for the first time since 2002 and his lowest ranking since 2003, Federer arrived in London tipped as favourite after winning back-to-back indoor titles in Basel and Paris.

The way he began against Tsonga did nothing to suggest otherwise, although he was helped by his opponent's errors.

Tsonga played a horrible fourth game to drop serve to love and hand Federer control and when the Swiss won 12 points in a row to take the opening set and move ahead in the second, fans in the darkened arena would have begun to feel a little short-changed at the one-sidedness of the match.

"In the first set I didn't put one ball in court," Tsonga, beaten by Federer in the Paris Masters final, told reporters. "I think he was a bit surprised because I played so badly in the first set, then I played correctly.

"I'm just disappointed because I didn't play like that the whole match."