By Julian Linden

NEW YORK (Reuters) - Kim Clijsters needed all her fighting qualities to keep her U.S. 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 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 7-6, sealing his win with a spectacular winner that went round the net post.

The prize for his grueling 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 U.S. 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.

The second seed had cruised through her first four matches this year in straight sets but found herself in deep trouble against Stosur when she began struggling with her serve in the swirling wind and admitted she was lucky to escape.

"I still didn't play a good match but I was obviously able to win it," she said.

"At the end of the day, what we try to do out here, is try to win the matches whether you play good or bad."

The Australian, who made the French Open final in June and saved four match points in her fourth round win over Elena Dementieva, led 3-2 in the final set before her own serve failed her and Clijsters won the remaining four games to seal victory.

"I'm happy to make it to the quarters, by far my best result here (but) I'm gonna be disappointed about this one," Stosur said.

"You can't break serve and have chance after chance after chance and blow it in a quarter-final.

"It's just too big of an opportunity to kind of let go, and unfortunately that's what I did today."


Williams, promoted to third seed this year after her sister and world number one Serena withdrew with an injury, provided a dazzling display that matched her sparkling pink dress to reach the semis at New York for the eighth time in 12 years.

"I feel like when the stakes were higher I was able to raise my game," Williams said. "She did, too, she played some great points. She's just so feisty that you have to kind of keep her at bay. I was glad I was able to do that."

"It was a very tough match and after losing the first two sets it was the best comeback of my career," Verdasco said.

Querrey's agonizing defeat left the United States without a quarter-finalist in the men's draw for the second year in a row.

"I left it all out there today and I didn't get to the quarters," said a deflated Querrey.

"I'm bummed, but I did everything I could."

(Editing by Ian Ransom)