London, United Kingdom (AFP) — Boris Becker has criticised world number two Andy Murray after the British tennis star voiced suspicions some of his opponents may have been taking performance-enhancing drugs.

Murray has been vocal in condemning the use of drugs in sport and enthusiastically backed the suspension imposed on leading female player, Maria Sharapova following her failed test for the banned substance meldonium at this year’s Australian Open.

More controversially Murray, beaten by Rafael Nadal in Monte Carlo on Saturday, has also spoken about being suspicious of opponents who he thought were not tiring as they ought to in matches.

But six-time Grand Slam winner Becker, now the coach of world number one Novak Djokovic, said Murray was “out of order” in making his feelings known without proof.

Becker, speaking at the Laureus World Sport Awards in Berlin, told Britain’s Daily Mail newspaper: “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,” the German added.

“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, the Daily Mail‘s sister paper: “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.”