Nadal's comeback saved him from experiencing the same result as second-seeded Andy Murray, who exited with a lifeless performance in a 7-5, 6-2, 6-2 loss to 16th-seeded Marin Cilic of Croatia. Murray, who was a finalist here last year, called it the "worst match of his life." And with those results, Nadal moves ahead of Murray and back to the No. 2 spot in the ATP computer rankings.
In the first night match, Flavia Pennetta hit lovely lobs that caused Serena Williams to reach for the sky and scream in anguish as they dropped over her head. Pennetta would engage Williams in double-digit stroke rallies and hit topspins and slices and win a point that would cause the crowd to marvel.
And then Williams would bounce the tennis ball once, twice, three times, toss it and boom. There would be an ace, or a service winner, timed at 116 mph, point won in a split-second.
Williams, seeded second and the U.S. Open defending champion, beat 10th-seeded Pennetta, 6-4, 6-3.
"Overall, this was a totally different match because she's a really consistent player," Williams said of Pennetta. "It was good."
Pennetta gave Williams her due. "She was playing really great," Pennetta said. "You have to play always the best tennis against her, and you cannot make a mistake. Today I made a few mistakes on important points, and she just took the chances."
The victory moved Williams into the semifinals, where she will play Kim Clijsters, who eliminated third-seeded Venus Williams on Friday in the fourth round.
"She plays tough. She plays hard," Serena Williams said of Clijsters. "And now it's a totally different level because she has nothing to lose. That's when you can play your ultimate best tennis."
The unseeded Clijsters won her 12th straight match at the Open, 6-2, 6-4, over 18th-seeded Li Na of China.
Cilic will play sixth-seeded Juan Martin del Potro in the quarterfinals. Del Potro, of Argentina, beat 24th-seeded Juan Carlos Ferrero of Spain, 6-3, 6-3, 6-3. Nadal will get 11th-seeded Francisco Gonzalez of Chile. Gonzalez took out seventh-seeded Jo-Wilfried Tsonga of France, 3-6, 6-3, 7-6 (3), 6-4.
Clijsters hadn't played here since she won her only major tournament title in 2005.
Since then, the 26-year-old from Belgium suffered through a number of serious injuries before taking a two-year break from tennis. She married former Villanova basketball player Brian Lynch and then gave birth to daughter Jada.
But Clijsters has played at this U.S. Open as if she has been out practicing every day of her retirement. She's been unnervingly accurate with her ground strokes and unfailingly supported by the U.S. crowds who gave her another standing ovation today at Arthur Ashe Stadium.