Djokovic warns his friend Andy Murray: You've got to be stronger

Both were considered young stars with bags of talent and promise who, if all continued to go well, would be challenging for Grand Slam titles in the future.

Take it from me: Djokovic has timely advice for Murray

Now, while Djokovic has made his breakthrough at the highest level, winning his first major title at the Australian Open in Melbourne in January, Murray's career appears to have stalled.

But Djokovic, who turned 21 last Thursday - a week after Murray - is convinced that Britain's No 1 will fulfil his vast potential, although he points to the Scot's physical and mental strength as areas he believes need improvement.

Djokovic said: "I still believe Andy has great potential and talent, but maybe he needs to improve his confidence.

"Last year, he got injured in Hamburg and missed Wimbledon on his favourite surface. He came back to win Doha early this year, but then he struggled a little. I think it's just a matter of time for Andy, but he needs to improve his physical and mental strength."

The Serb's progress through the rankings has been boosted by his success in the Grand Slams. Djokovic featured in semi-finals at Roland Garros and Wimbledon and was runner-up to Roger Federer in New York last year before beating the Swiss superstar in the Australian semi-finals on his way to ultimate victory.

That has seen him move ever closer to Rafal Nadal's No 2 world ranking, while Murray has battled to claw his way back to No 11.

But Djokovic is convinced Murray can join him in the top five. "There's still a lot of time on Andy's side to come and join me," he said. "But he needs to start winning again."

That is an art Djokovic has fine tuned, and he approaches the French Open, which begins at Roland Garros today, with his confidence so high that he is happy to place a question mark over the future of world No 1 Federer and his ability to surpass the record of 14 Grand Slam titles won by Pete Sampras.

"This time last year I would have said Federer would beat Sampras's record. Now I'm not so sure," said Djokovic. "His aura has gone. He's not as dominant as he was, and since I beat him in Australia he's looked frustrated. Players are beginning to challenge him now, especially myself and Rafa. He's got 12 Grand Slams to his name and maybe he will beat Sampras, but now I'm here it will be tough for him."