|Twenty-fifth seed Wawrinka celebrates the biggest upset of the US Open so far as he sends Murray to the exit [AFP]
Andy Murray hobbled out of the US Open as Switzerland's Stanislas Wawrinka beat him in a third-round match in which both players struggled with injury.
Wawrinka won 6-7 7-6 6-3 6-3 on Sunday as the Scotsman struggled to run at full speed after having treatment to his legs, elbow and neck.
The fourth seed was powerless to stop Wawrinka 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 was another disappointment for the Scotsman, who arrived at Flushing Meadows seeded fourth, in great form, and holding genuine hopes of ending Britain's 74-year wait for a men's grand slam singles champion.
"I'm very disappointed, obviously, but I think I've been more disappointed," said Murray, the 2008 runner-up.
"In other grand slams, when you get closer to winning the tournament, it becomes a lot harder to take."
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 Rafael Nadal, who was drawn to face Murray in the semi-finals.
Nadal, playing before Murray's defeat, sent a warning to his rivals about his intentions to win the US Open and complete his collection of grand slam titles.
"I think I've been more disappointed...when you get closer to winning the tournament, it becomes a lot harder to take"
Andy Murray, defeated US Open fourth seed
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.
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.
"It's always good to have a tougher match, a kind of a match where you have to challenge yourself against your opponent and the conditions," said Williams.
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.
Stosur saves four
Like Nadal and Williams, she is yet to drop a set in the championship and will face fifth-seeded Australian Samantha Stosur, who saved four match points in a thrilling third set and came back to beat Russian Elena Dementieva 6-3 2-6 7-6.
Stosur won the climactic tiebreaker 7-2 to end the titanic, two-hour 38-minute match in the latest-ending women's match ever staged at the US Open.
Italy's Francesca Schiavone, brimming with confidence after winning her maiden grand slam title at this year's French Open, also cruised through to the last eight, beating Russia's Anastasia Pavlyuchenkova 6-3 6-0.
Schiavone will play Williams in Tuesday's quarter-finals.
Spain are already assured of at least one men's semi-finalist after Fernando Verdasco, David Ferrer and Felciano Lopez joined their countryman Nadal in the same section of the draw.
Nadal will face Lopez next with the winner to play either Ferrer or Verdasco.
Another two Spaniards, Albert Montanes and Tommy Robredo, are also through to the last 16, giving Spain a total of six, easily the most by any one country this year.