Rafael Nadal started his latest challenge for a first ATP World Tour Finals title with a 6-2, 3-6, 7-6 (7/3) victory over America's Mardy Fish on Sunday. World number two Nadal, who has won 10 Grand Slams in his career, would love to add the ATP's end of season finale to his trophy cabinet but he will need to improve on this hard-fought win in his opening Group B fixture. Although Nadal, beaten by Roger Federer in last year's final here at the O2 Arena, has reached three Grand Slam finals this year, winning the French Open for the sixth time, he will still look back on the last 12 months as an unfulfilling time. The sole reason for that is Novak Djokovic, the remarkable Serb who defeated Nadal in the Wimbledon and US Open finals, as well as four Masters finals, and replaced him as world number one. The 25-year-old can erase some of that frustration by taking the title here and his gritty victory means he joins Federer, who defeated Jo-Wilfried Tsonga earlier on Sunday, on top of Group B after the opening round of matches. After spending most of his career swimming in the sport's back waters, Fish, 29, has finally cracked the world's top 10 this year and was making his first appearance in the Tour Finals. Initially he seemed affected by the occasion and he was broken in the first game of the match as a pair of wayward ground-strokes and two tense volleys gifted Nadal the initiative. Nadal had no problems holding onto that advantage and it wasn't long before he started unloading some vicious top-spin forehands. Fish was broken again as Nadal produced two stunning cross-court winners to take a 5-2 lead before serving out the set. Nadal seemed to relax a little too much after that and Fish, his nerves finally under control, was able to punish him with a break for a 2-0 lead in the second set. Fish's serve was proving a more dangerous weapon and, combined with some clever ground-strokes that belied his hulking frame, the American was able to change the momentum of the match. Nadal fought off four set points on his serve at 2-5 but Fish repelled a break point on his own serve in the game to force a final set. A sublime forehand winner on the run from outside the tramlines gave Nadal a break in the second game of the decider, but Fish responded with a perfect drop shot to break straight back. Fish won another break with a backhand winner from the baseline, however the American couldn't push home his advantage and allowed Nadal to break again in the next game. Nadal squandered two match points at 4-5 and a tie-break was needed to settle the match. He may have been puffing hard by now but Nadal had just enough energy to get over the line as Fish's forehand into the net gave him the decisive mini-break.