Nadal serves up a timely treat for his uncle Toni

With less than a week to go before the US Open, Rafael Nadal was concerned. His backhand had been "terrible", he had been making too many unforced errors on his forehand and his serve had been ineffective. Opponents were regularly driving their returns to his backhand as the Spaniard tried to run round the ball and avoid playing it on his weaker flank.

After only two matches at Flushing Meadows, the world No 1's game had been turned around. He had not lost a set or dropped his serve and was striking the ball with more confidence, especially on his backhand. Everything had changed with the arrival in America of his uncle and coach, Toni Nadal, in the week before the tournament.

"Toni arrives and everything is under control," Nadal explained with a laugh after his second-round victory over Denis Istomin. "I started serving well one or two days before the competition, but in practice the week before it wasn't good.

"I changed the grip a little bit five or six days ago because I felt when I played against the wind I wasn't getting any free points. I tried to serve with a little bit more aggression. For the moment it's working really well so I'm going to try to keep playing like this. Serving like this gives me big confidence in my game."

Less than a fortnight later, Nadal became US Open champion and only the seventh man to win all four Grand Slam titles. In seven matches he dropped his serve only five times, a performance that only Andy Roddick has matched since such statistics were first compiled in 1991.

The extra power in his serve, helped by the change in grip, gave Nadal the confidence to serve with more variety and play with more aggression. His early successes on clay derived from his ability as a counter-puncher, but on other surfaces he has learned the need to attack more. Four of his last five Grand Slam titles – he has won nine in total – have been won on grass or hard courts.

Nevertheless, it was an extraordinary performance given both Nadal's hard-court form earlier in the summer – he lost to Andy Murray in Toronto and to Marcos Baghdatis in Cincinnati – and the conditions at the US Open. Many experts said the Mallorcan's heavily spun forehands, cracked with a huge arcing swing, would make it difficult for him to cope with the pace of the "unfluffy" Wilson balls off a surface that some consider to be quicker even than Wimbledon's.

Although Nadal agreed that his serve "probably made the big difference", he insisted after his victory that he had made no significant changes to his hard-court game, which he said was much as it had been when he won last year's Australian Open.

"Part of it is confidence," he said. "When you are playing well and your confidence is high, it seems like you've improved a lot, but there are moments when you're not playing that well, when you lose your confidence, you lose matches and it seems like you've forgotten how to play tennis. It's not like I've improved a lot since 2009. I think I've improved my tennis a little bit, but it's not a radical change."

Much of that confidence derives from his relationship with Toni, who has been his coach since he was four. When you watch their practice sessions, the respect and affection between nephew and uncle is clear.

Rafael, a tireless worker, plays and practises with unrelenting commitment. He said: "My goal all my life has been the same: to keep improving and make myself feel a better player next year than what I felt this year."

What does he see as his greatest strengths? "I think my mentality, my attitude on court has always been good. I am positive on court and I fight all the time. But that's not the only thing. I think I was able to listen all the time to my coach, to make adjustments and to be ready to change things in order to improve.

"If you're talking about my game, I think the best thing that I have is my intensity on court. When I am playing well, the intensity always is high. The rhythm is high. Sometimes you can hit easy winners, but I can hit winners after three or four shots by keeping a great rhythm all the time."

Nadal is already certain to finish the year as world No 1. Given that he has no titles to defend until next April, he is also likely to establish the biggest lead in the history of the world rankings. His current advantage of 4,880 points over Novak Djokovic, the world No 2, is only 310 short of the record, which Nadal himself established in May 2009, ahead of Roger Federer.