Andy Murray pinpointed his improved fitness as a key reason for his success after he became the first Briton to win the Sony Ericsson Open with a straight-sets victory against Novak Djokovic.

The 21-year-old was in superb form in Miami, dropping just two sets on the way to the final which he convincingly won 6-2, 7-5, despite the blistering heat.

Whereas the draining conditions may have slowed Murray in previous years, he looked unfazed as he secured the 11th title of his career and put himself on the brink of passing Djokovic into third place in the world rankings.

Since his defeat to world No1 Rafael Nadal in last year's quarter-finals at Wimbledon, Murray has had a win-loss record of 57-7 - the best on the ATP tour ahead of the Spaniard's 50-7. And the Scot believes his sparkling form is down to him being better prepared physically. Murray said: "It just gives you more confidence going into the matches knowing you're in good shape. I think just mentally it makes a difference. Even if you're struggling, you know your opponent is going to be feeling the same, as well.

"Whereas before sometimes you could get tired and look over at the other side and the opponent seems fine. A match like today, there's obviously points . . . I mean, it was hot out there.

"A few long rallies and stuff I would be a little bit out of breath. I could look down the court and see him struggling, as well."

This was Murray's second tournament back from a virus and the impressive manner of his return - he was runner-up at Indian Wells - provides further evidence of his new-found strength.

The beaten Djokovic is certainly impressed. "I think physically he's moving much better all over the court," said the Serb.

"The balls he wasn't getting before, he is now. The switch of defensive to offensive tennis in the points - he is really good, one of the best in the world.I think that makes him a much better player than he used to be."

With the hard-court season now over, Murray is focusing on trying to master the clay courts ahead of next month's French Open.

Murray admits he still has some way to make up to compete with Nadal, Djokovic and Roger Federer on that surface.

"The clay-court season will be very important for me now," said the British No1."On hard courts, my game is up there with the others but on clay it hasn't been the same. If you take the results on it out of the equation, I'm very close to Roger and just behind Rafa, so I need to improve."