Stanislas Wawrinka of Switzerland exults after kicking Andy Murray of Great Britain out of the US Open. (Reuters)

NEW YORK (Reuters) — Andy Murray hobbled out of the US Open on Sunday, beaten 6-7, 7-6, 6-3, 6-3, in the third round by Switzerland's Stanislas Wawrinka after his ailing body let him down.

Struggling to run at full speed after requiring treatment to his legs, elbow and neck, Murray appeared powerless to stop Wawrinka, who was battling his own injury problems but still played superbly, from pulling off the biggest upset of the championship.

''I was struggling physically and I got frustrated with that,'' Murray said.

''Maybe I felt like my chance of doing well here was slipping away. I've worked very hard to give myself a chance of winning tournaments.

''It happens to everyone in life at different points.''

Wawrinka, seeded 25th, advanced to play Sam Querrey, the only American left in the men's competition, in the fourth round although the big winner could well be Rafa Nadal, who was drawn to face Murray in the semi-finals.

Nadal, playing before Murray's defeat, sent an ominous warning to his rivals about his intentions to win the US Open and complete his collection of grand slam titles.

The Spaniard, getting better with each match on the back of a new and improved serve, overpowered France's Gilles Simon 6-4 6-4 6-2 to steam into the fourth round without dropping a single service game in three matches.

Venus Williams and Kim Clijsters passed their first real tests with flying colours to safely book their places in the women's quarter-finals.

The pair could hardly have been more impressive as they swept past tricky opponents to remain on course for a blockbuster semi-final showdown when the season's last grand slam enters its second week.

Williams, the champion at Flushing

Meadows in 2000 and 2001, defeated Israeli 16th seed Shahar Peer 7-6 6-3 in an ideal preparation for the more difficult matches ahead of her.

Clijsters, who won the title in 2005 and again last year, thrashed Serbia's former world number one Ana Ivanovic 6-2 6-1 in just under an hour. Like Nadal and Williams, she is yet to drop a set in the championship.