Boris Becker hits back at Andy Murray’s claims over doping

The British No 1, who lost to Rafa Nadal in the semi-finals of the Monte Carlo Masters on Saturday, has been one of the biggest critics of drugs cheats in sport and has regularly called on authorities to do more to clamp down on cheats.

And the 28-year-old revealed his reservations about some players during matches in the past, saying that some of them did not "seem to be getting tired".

But Becker, who coaches world No 1 Novak Djokovic, has now hit back. Speaking at the Laureus World Sport Awards, the German told the Daily Mail: "We have random drug-testing and unless it's proven, they are 100 per cent innocent. So to assume something because somebody has won a Grand Slam or is fitter is totally out of order.

"Andy is one of the fittest players on the tour - he often outlasts players and nobody is questioning his ethics.

"I believe 100 per cent Andy is clean. Roger [Federer] is clean, Rafa is clean, all these guys are clean. Novak gets tested a lot. That can mean twice in a Grand Slam."

Murray had told the Mail On Sunday: "I have played against players and thought, 'They won't go away' or 'They don't seem to be getting tired. Have I ever been suspicious of someone? Yeah. You hear things.

"It's harder to tell in our sport as people can make big improvements to a stroke or start serving better because they have made technical changes.

"If it's purely physical and you're watching someone playing six-hour matches over and over and showing no signs of being tired, you'd look at that."

However, Murray also insisted that the provisional ban on Maria Sharapova, after she tested positive for taking meldonium at the Australian Open, shows that tennis is not ignoring the issue.

"When someone like Sharapova is banned, I see that as being a positive," he added. "If that stuff is happening and you don't hear about it, I have a big issue because it's like the sport is covering up for the big stars."