NEW YORK (Reuters) – Kim Clijsters needed all her fighting qualities to keep her US Open defense alive and cap an extraordinary day of titanic struggles at the last grand slam of the season Tuesday.

Now unbeaten at Flushing Meadows in 19 matches dating back over five years, the Belgian regained her composure when she needed it most to reel off four games in a row and defeat Australia's Samantha Stosur 6-4, 5-7, 6-3 in an error-prone but absorbing center court clash.

She now faces Venus Williams in Friday's semi-finals after the former world number one kept the host nation in the championship with a 7-6, 6-4 win over French Open champion Francesca Schiavone.

The United States' hopes of winning the men's title were extinguished when Sam Querrey lost a four and a half hour armwrestle with Stanislas Wawrinka 7-6, 6-7, 7-5, 4-6, 6-4, the first of two five-setters on a marathon eighth day that did not finish until 1:16am (0516 GMT) Wednesday morning.

The Spanish pair of Fernando Verdasco and David Ferrer also slugged it out at the Louis Armstrong Stadium for nearly four and a half hours before Verdasco emerged victorious 5-7, 6-7, 6-3, 6-3, 7-6, sealing his win with a spectacular winner that went round the net post.

The prize for his gruelling win is a quarter-final showdown with world number one Rafa Nadal, who restored some normality to the day by beating Feliciano Lopez 6-3, 6-4, 6-4.

Nadal has never made it past the semi-finals in New York but is in devastating form this year, winning his four matches in straight sets without dropping a single game on service.

“I think sometimes it's part of the confidence,” Nadal said in a courtside interview. “It's true I'm serving faster than ever in every tournament. We will see what happens now.

“This year I'm healthy I hope, continuing playing well and have my chance in the next match.”

Wawrinka, who upset Andy Murray in the third round, faces 12th seed Mikhail Youzhny next after the Russian outclassed Tommy Robredo, one of five Spanish men in action, 7-5, 6-2, 4-6, 6-4.

Clijsters provided a fairytale ending to last year's US Open when she returned to the game after taking time out to start a family and won the championship four years after her only previous grand slam success