ANDY MURRAY winning the French Open would be more colossal than when he was crowned Wimbledon champion.

That's the belief of Great Britain's Davis Cup captain Leon Smith, who has backed his friend and fellow-Scot to continue his impressive run on clay.

The world No.3 is unbeaten on the surface this season, having won the Munich Open, the Madrid Masters and one game in Rome as well as his opening match at Roland Garros.

It is easily the best form Murray has ever shown on the dirt, the title in Germany last month being a first for the the British No.1.

Murray's greatest achievement so far is undoubtedly lifting the trophy at the All England Club in 2013, with becoming US Open champion and an Olympic gold medallist in 2012 also high on his list of achievements.

But Smith reasons that claiming the French Open might in some ways be even more significant.

"You could argue that," said the Glaswegian. "Andy getting over the line at Wimbledon - given it is steeped in history, it's his home event and the expectation to win it given the whole Fred Perry story - that was a whole different kind of pressure.

"But for a British player not brought up on this surface to win the French Open would be immense, just huge.

"And probably in terms of an achievement it would top Wimbledon and anything Andy has done before, given the magnitude of winning on the surface and the environment of the French Open."

It is still early days for Murray in Paris, but the signs are already promising. Building on his success in the last two months on clay, he got his French Open bid off to a comfortable start on Monday with a 6-3, 6-3, 6-1 victory over Argentina's Facundo Arguello.

His next opponent will be Joao Sousa of Portugal tomorrow, and Smith has pointed to Murray's excellent preparation as the key to his stunning form so far on clay, which now stretches to 11 straight wins.

"He has clearly had a very good clay court season so far," Smith said. "With his wedding he obviously didn't play at Monte Carlo but after that he did a training camp in Barcelona with his new coach Jonas Bjorkman.

"That was a good camp for him as it allowed him to prepare on the surface as Andy, as he says himself, maybe takes a bit longer to adjust than players who were brought up on it. He really only trained on it regularly at 15 or 16 when he moved to Spain.

"That helped, but he's really just had a dream start to this year's clay season. You have the wins in Madrid and Munich, and he's playing great tennis while moving very well, which is such an important element of his game.

"Much of Andy's success comes from how well he gets about the court. When you add in his improved serving and the confidence this run has given him, it makes him look really good on the surface."

Speaking to SportTimes last year, Smith tipped Murray for big things in 2015, and so far his old friend has been spot-on. Since January, the Scot has reached the Australian Open final, been the hero in Glasgow for the British Davis Cup team against America, snared two clay-court titles and seen his world ranking rise back up to three.

"We saw how well Andy was playing in Melbourne at the Australian Open," Smith said. "I thought he was exceptional, including in the final, even if it didn't go his way.

"The whole fortnight was brilliant and it's just carried on from there. Andy was great in the Davis Cup in Glasgow and I think playing in the competition always helps him. He enjoys it so much and it always gives him a boost between the major events.

"He's had to work hard. He maybe didn't play the way he wanted to at the tail end of last year, but he was winning matches by working so hard.

"Andy had to dig deep and that helped him through the pre-season and really showed in Melbourne. It's really looking like Novak Djokovic and Andy are the players to beat in 2015."