Lacklustre Nadal has to grind out victory

Rafael Nadal had warned before the start of the Barclays ATP World Tour Finals that the conditions suited him less than all his main rivals, but the 25-year-old Spaniard is unlikely to have envisaged an almighty struggle to beat the rank outsider in the eight-man field at these season-ending championships.

Nadal ultimately showed all his battling qualities to beat Mardy Fish 6-2, 3-6, 7-6 after two hours and 53 minutes at London's O2 Arena, but it was not the performance you would expect from the world No 2. His next match in the round-robin stage will be tomorrow against his old rival, Roger Federer, who beat Jo-Wilfried Tsonga 6-2, 2-6, 6-4 in his opening match.

Fish is making his debut in the end-of-year finale at the age of 29. The American was one of the last to qualify, is the lowest-ranked player in the field at No 8 and has been struggling with a hamstring injury that forced him to retire mid-match in his last two tournaments. He had also lost seven of his eight previous meetings against Nadal.

Perhaps Nadal was paying for his ring-rustiness, having chosen to miss the recent Paris Masters. He dashed off court for a toilet break early in the third set and said afterwards that he had been suffering from a stomach problem for the last hour of the match. "I felt very bad from the start of the third set," he said. "It was a very important match for me, for my confidence."

Nevertheless, the Spaniard appears to be finishing the year in a significantly weaker state than he began it. Six successive defeats in finals to Novak Djokovic – the man who replaced him at the top of the world rankings – during the course of the season appear to have dented his self-belief.

Nadal has a reputation as a great front-runner, but his response to winning the first set in just 34 minutes against a nervous-looking Fish was curious. Errors quickly crept into the Spaniard's game, boosting Fish's confidence.

An attacking player who loves to get into the net, Fish started serving with greater conviction, hit some smart volleys and showed good touch with nicely judged drop shots. The American, giving Nadal less time on his ground strokes, levelled the match on his fifth set point.

There were four breaks of serve in the first six games of what became a thrilling decider. Fish saved two match points in the 10th game, but Nadal never trailed in the tie-break, which he won 7-3 when Fish put a high backhand in the net. By the time the match finished at 11.29pm there were banks of empty seats in the 17,500-capacity arena, many spectators having left in order to ensure they did not miss their last buses or trains home.

Andy Murray's group play their first matches today, with the world No 3 facing David Ferrer this afternoon and Djokovic meeting Tomas Berdych in the evening. Murray has only ever lost to Ferrer on clay and beat the world No 5 in straight sets in the group stage here 12 months ago.