Nadal's Fitness Under Microscope at U.S. Open

The forecast of heavy rain at the U.S. Open for Tuesday may come as a welcome relief to Rafa Nadal after his health scare at the last grand slam of the year.

The defending champion collapsed during his post-match news conference on Sunday because of severe cramping in his right leg after he had beaten David Nalbandian on a hot and humid afternoon at Flushing Meadows.

But the prediction of cooler weather and possibility of showers should make life easier for the Spaniard when he meets Gilles Muller of Luxembourg for a place in the quarter-finals.

Muller, who earned his place in the last 16 with a straight sets win over Russia's Igor Kunitsyn, is one of two unseeded men in the left in the bottom half of the men's draw.

The other is American wildcard Donald Young who faces British fourth seed Andy Murray in a tantalizing encounter on the Arthur Ashe Stadium.

Young beat the Scotsman in their only previous meeting earlier this year and said the result gave him a real boost of confidence.

At that point it was the biggest win in my career, Young said about their encounter at Indian Wells.

Murray, who survived a scare in his second round match when he came from two sets down, but won his third round match in straight sets, expected he would improve more against Young.

Things can change so much in a couple of days so you just need to make sure you stay focused on the next match, Murray said.

It's a big match for me because obviously what happened earlier in the year, but I think energy wise I'll be feeling good going into that one hopefully for a long second week.

Young is one of three Americans playing in the fourth round on Tuesday. Former U.S. Open champion Andy Roddick faces Spanish fifth seed David Ferrer, while big-serving John Isner takes on Frenchman Gilles Simon, who beat 2009 champion Juan Martin Del Potro in the third round.

The women's draw, a day ahead of the men, goes straight into the quarter-finals with two intriguing clashes on Louis Armstrong Stadium.

Germany's Angelique Kerber, the only unseeded woman left, faces Italian giant-killer Flavia Pennetta, who knocked out Maria Sharapova in the third round.

Russia's Vera Zvonareva, last year's runner-up and the second seed this time, faces Australia's Sam Stosur, who has developed a reputation as the ironhorse of the competition after winning back-to-back marathon matches.

I'm really proud of myself for getting through these two matches, said Stosur, who has won each of her past seven matches with Zvonareva.

I haven't always been known for my competitiveness out there, to really fight hard, but for it to come out back-to-back days, it's definitely very rewarding. I know now I can do it.