Caroline Wozniacki of Denmark celebrates during the fourth round match of women's singles against Svetlana Kuznetsova of Russia at the US Open tennis championships in New York, the United States, Sept. 6, 2011. Wozniacki won with 6-7 (6/8), 7-5, 6-1. (Xinhua/Shen Hong)
Wozniacki rallied to down No. 15 seed Kuznetsova of Russia, 6-7 (6), 7-5, 6-1 to make it to the quarterfinals over a match lasting just over three hours.
Three-time champion and former world No. 1 Serena Williams secured a berth in the quarterfinals much easier as the 28th-seeded American handled 16th-seeded Serb Ana Ivanovic 6-3, 6-4.
Wozniacki was behind 4-1 in the second set as the match went deep into the night at Arthur Ashe Stadium.
"I thought about the match we played two years ago. I was down 4-1 in the second set as well," Wozniacki said.
"I just stepped into the lines a little bit and went for my shots a bit more and made a few."
Two years ago, the Danish bounced back for a three-set win in the round-of-16, capturing the final two sets in tiebreakers.
Wozniacki, the runner-up in 2009, won four straight games during the second set and capitalized numerous errors from her opponent.
Kuznetsova missed an overhead slam during the four-game run, and when her forehand went long, Wozniacki forced the decisive third set.
She was dominant in that final set and fortunate as well to win the match, thanks mostly to 78 unforced errors from Kuznetsova, the 2004 U.S. Open champion and runner-up in 2007.
Wozniacki set up a quarterfinal match against 10th-seeded German Andrea Petkovic, who ousted Spain's Carla Suarez Navarro, 6-1, 6-4.
Williams, who showed formidable form at Flushing Meadow by winning fourth round matches all in straight sets, jumped to a a quick 3-0 lead in the first set before the former No. 1 Ivanovic fought back to pull even at 3-all.
The Serb then had a break-point chance to grab a 4-3 edge in the stanza, but the American stalwart saved it on her way to winning the last three games of the set.
The 29-year-old Williams cruised through in the second amid windy conditions to reach her ninth career US Open quarterfinal. Ivanovic's 29th unforced error put an end to the 75-minute play.
Ivanovic is now 0-4 in fourth-round Grand Slam matches since capturing the 2008 French Open title.
"My serve broke down little more than hers did," Ivanovic said. "I still created lots of opportunities and I felt I was stepping up a lot, and I just felt that was the biggest difference today."
Williams is now riding a 16-match overall winning streak, all on hardcourts, including titles in Stanford and Toronto. She's 20-2 this season since returning to action in June following an 11-month injury.
Williams, who owns 13 career Grand Slam titles, including three here at Flushing Meadows, will face 20-year-old Russian Anastasia Pavlyuchenkova in the round of eight.
The 17th-seeded Russian upset the seventh-seeded Schiavone, former French Open champion, 5-7, 6-3, 6-4.
Pavlyuchenkova prevailed after she hit a big forehand winner on her third match point after two hour and 41 minute contest, featuring 16 service breaks and 21 double faults. ' "I tried not to let her play her game, because she's very smart and she makes me run a lot," Pavlyuchenkova said after toppling Schiavone.
The 31-year-old Schiavone outlasted Pavlyuchenkova in a French Open quarterfinal back in June, but not again as the Russian now is to appear in her first-ever US Open quarterfinal.