Playing for the sixth year at Flushing Meadows, the No. 1-ranked Nadal on Monday rallied to reach the quarterfinals.

He got quite a workout - and a scare - from his unseeded, 20-year-old American opponent.

"Very tough," Nadal said. "Sam is a big player, a big server. He has a great future." Down 6-5 in the third set, Nadal suddenly flashed the form that won at Wimbledon, the French Open and the Olympics.

Hitting harder, grunting louder and moving in closer, he quickly forced a tiebreaker and leaped to celebrate.

Nadal won the final four points to take the tiebreaker and later served out the match.

Nadal next plays another unseeded American, Mardy Fish, who reached the quarterfinals at the US Open for the first time with a 7-5, 6-2, 6-2 win over No. 32 Gael Monfils.

Also advancing was No. 17 Juan Martin del Potro of Argentina, who ended the run of Kei Nishikori, the first Japanese man to reach the U.S. Open's fourth round in the 40-year Open era. Del Potro won the contest between teenagers 6-3, 6-4, 6-3 for his 23rd consecutive victory.

Del Potro will face No. 6 Andy Murray of Britain or Swiss No. 10 Stanislas Wawrinka, who were playing Monday night's last match.

Venus and Serena Williams set up yet another Grand Slam family reunion. Two months after meeting in the Wimbledon final, this time they square off in the quarterfinals.

Venus beat ninth-seeded Agnieszka Radwanska of Poland 6-1, 6-3, and Serena dispatched wild card Severine Bremond of France 6-2, 6-2.

Both sisters are two-time US Open champions. They are 8-8 overall against each other, including 5-5 at majors. Their matchups, however, usually come much later than the quarters.

"I mean, it's tough to play her because she's so good," Venus said. "We're going stronger than ever, in my opinion."

Also, sixth-seeded Dinara Safina beat qualifier Anna-Lena Groenefeld of Germany 7-5, 6-0, and will face No. 16 Flavia Pennetta, who defeated two-time major champion Amelie Maursemo 6-3, 6-0.

Ranked 55th, Querrey gave a solid show and became the first player to take a set from Nadal at this Open. His fans in the Arthur Ashe Stadium private box were vocal, particularly the three who had their shirts off and spelled out S-A-M on their chests.

A bit jittery at the start, the 6-foot-6 (1.98-meter) Querrey boomed 20 aces and showed no fear, trading big shots with Nadal all match long. He drew a standing ovation at the end for his effort, and smiled broadly.

"Came up a little short. The guy's too good," Querrey said.

Playing in his ninth US Open and coming off his upset over James Blake, Fish kept busy as his wedding approaches. He will marry "Deal or No Deal" model Stacey Gardner on September 28 - she watched from his private box, minus her shiny No. 2 briefcase.

"I desperately wanted to play well here," he said.

"Desperately."

Fish matched the best Grand Slam result of his career, having reached the quarters at the 2007 Australian Open.

Ranked as high as No. 17 in 2004, the 26-year-old Fish has worked three years to recover from an injured wrist that required extensive surgery.

Fish waved to his private box after match point. The entourage included his father, Tom, who celebrated his birthday. No such fun for Monfils, who turned 22 on Monday.

Safina coasted home after a close first set against Groenefeld, the first qualifier to reach the fourth round since Anna Kournikova in 1996. Safina equaled her best result at Flushing Meadows, and kept up the best season of her career.