Two-time Australian Open champion Victoria Azarenka advanced to the US Open quarter-finals on Tuesday, beating Serbian 13th seed Ana Ivanovic 4-6, 6-3, 6-4 in a match postponed from Monday due to rain.
The 24-year-old second seed from Belarus took advantage of Ivanovic's 42 unforced errors and broke the former world number one nine times in the match, winning despite surrendering seven breaks herself and hitting just 21 winners.
Ivanovic had fired 15 forehand winners before Azarenka managed her first.
"Ana just came out firing. She played really amazing tennis in the first set," Azarenka said. "I was sleeping and she was ripping winners.
"I had to wake up in the second set. I was more determined and focused and took advantage of those opportunities."
Azarenka, who lost to Serena Williams in last year's US Open final, will play for a semi-final spot against Slovakia's 48th-ranked Daniela Hantuchova, with whom she has split four career meetings.
"She's playing the best tennis of her life," Azarenka said of Hantuchova. "She is really playing well and has a lot of confidence. We hit with each other a lot so there will be no surprises there."
Azarenka, a semi-finalist in five of the past seven Grand Slams, defeated world number one Williams in the final at Cincinnati in the last big US Open tune-up event to become the main threat to dethrone Williams.
Williams, a 16-time Grand Slam winner, and Chinese fifth seed Li Na, the 2011 French Open champion, will try to reach the semi-finals in later matches at Arthur Ashe Stadium.
Neither has dropped a set in reaching the last eight.
Ivanovic, the 2008 French Open winner, made her first Slam quarter-final since her Roland Garros crown at last year's US Open, but could not duplicate the effort.
After Ivanovic took an early break and served for the first set, Azarenka broke back in the ninth game, using an overhead smash on her third chance, to begin a run of seven consecutive broken serves.
Ivanovic double faulted away a break for her second service game in a row before Azarenka held to end the futility streak but Azarenka broke again in the ninth game to force a third set.
Azarenka broke for a 2-0 lead in the third set but Ivanovic answered with a lob winner to break back in the third game and they exchanged breaks twice more before Ivanovic held to pull level at 3-3.
Azarenka held and Ivanovic was broken again, her backhand wide allowed the Belarussian to serve for the match.
Ivanovic answered by saving two match points in the ninth game, Azarenka sending backhands beyond the baseline and into the net, and broke within 5-4.
"The match points were pretty bad," Azarenka said. "I really didn't go for my shots. I kept waiting for a mistake, which didn't really happen, so I had to go for my shot. She was making great shots."
But Azarenka broke back for the match, Ivanovic blasting a forehand winner to deny a third match point before netting a back to finally end matters after two hours and 10 minutes.
Reigning French Open champion Williams, seeking her fifth US Open crown, will face Spanish 18th seed Carla Suarez Navarro, who has never won a title.
The first Spaniard in the US Open quarter-finals since Arantxa Sanchez-Vicario in 1998 seeks her first Grand Slam semi-final in her first quarter-final appearance since the 2009 Australian Open.
Suarez Navarro, who has yet to take more than three games off Williams in two meetings, could become the first Spanish woman to upset the world number one since Virginia Ruano Pascual beat Martina Hingis at Wimbledon in 2001.
Li was set to meet Russian 24th seed Ekaterina Makarova with the winner advancing to her first US Open semi-final.
Makarova, who has never reached a Grand Slam semi-final, upset Williams in last year's Australian Open fourth round but has lost all three prior meetings with Li.
Either Williams, seeking her ninth title of the year, or Li, this year's Australian Open runner-up, would become the oldest women's champion in US Open history at age 31 with a triumph in Sunday's championship match.
The total of five players over 30 in the quarter-finals was a US Open record and matched the all-time Grand Slam mark from the 1977 Australian Open.