Andy Murray's clay-court credentials have been given a glowing endorsement as he bids to take his French Open campaign into the business end of the tournament.

Murray, the third seed, has made the fourth round at Roland Garros for the first time in his career and it seems he is finally getting to grips with the dirt - by far his least favourite surface.

The British number one has lost just one set in the first three rounds, which have seen him overcome Juan Ignacio Chela, Potito Starace and Janko Tipsarevic.

He next plays in-form 13th seed Marin Cilic, who believes Murray will be exceptionally hard to beat the way he is performing.

"He's one of the better movers on tour so it's really tough to play him because he always gets another ball back," said the Croatian.

"I also think he's pretty smart on court. He reads the game well and takes advantage of what he sees can help him. He's also serving well.

"So you need to be consistent and physically strong to maintain your power and to maintain your consistency when you play him."

Tipsarevic was forced to retire midway through his third-round clash with Murray, the Serb failing to recover from a hamstring problem he sustained early in the match.

He was 7-6 (7/3) 6-3 down when he threw in the towel but even when carrying an injury, he was 5-2 ahead in the first set and squandered two set points.

Despite feeling his hamstring complaint cost him a great chance of upsetting Murray, the world number 65 was effusive in his praise of the Scot.

"We're talking about a guy who is making the minimum amount of errors on the slowest surface in the world," Tipsarevic said.

"In my opinion, he's the best defender on tour and has the best passing shot on tour. He's not (Rafael) Nadal, but he's still a great player on clay.

"The most difficult thing is that he's playing the right shots in the important moments. It's probably because of the confidence he has got in the last couple of months - he makes you play that extra shot when another guy on tour will not do that."

Murray has won both of his matches against the 20-year-old Cilic as a senior, in a Davis Cup tie on grass in 2007 and on a hard court in Madrid last year.

That has given him confidence heading into his last-16 match, scheduled to be played tomorrow, but Murray is wary nevertheless.

"He's a very good player," the Scot said of Cilic. "He's young and he goes for his shots.

"He is playing well, he's got a big game and he's dangerous if he's confident. But I beat him the times we've played on the tour and I'll take that into the match."

Cilic is yet to lose a set this tournament and has dropped just 21 games in total.

"I haven't had such good results in the past few tournaments on clay but here I have found my form and am playing well. I'm enjoying it," he said.

"It's obviously not his (Murray's) best surface so I think I have a good chance to play good tennis and get in a good position to win the match.

"I think I'm physically prepared, and I'm going to be ready whatever happens."