Caroline Wozniacki won a late-night thriller against former champion Svetlana Kuznetsova at the US Open on Monday to stay on course for a heavyweight showdown with Serena Williams.
The world number one Wozniacki clawed her way back from the brink of defeat to win 6-7 7-5 6-1 and advance to the quarter-finals after a three-hour clash that ended shortly before midnight.
Her next match is against Germany's Andrea Petkovic after the 10th seed defeated Carla Suarez Navarro of Spain 6-1 6-4.
Williams, closing in on what could be the greatest achievement of her incredible career, looms as a likely semi-final opponent.
While Wozniacki struggled, the American stormed into the last eight by beating Ana Ivanovic 6-3 6-4 to establish herself as the overwhelming favourite for the title.
"It's really important for me just to look at the mountain and keep climbing it," said Williams.
Novak Djokovic is also edging towards the men's championship. The Serbian has been almost invincible this year and is through to the quarters without dropping a set, although he had to survive an epic tiebreaker to maintain the record.
The Serbian survived a 30-point classic against Ukraine's Alexandr Dolgopolov, winning 16-14, and pumped his first like he had won the final. He ran away with the next two sets to win 7-6 6-4 6-2.
"This is one of the longest tiebreaks I ever played," he said. "It was certainly exciting to be part of it but I knew that I needed to win that set."
Djokovic's next opponent is his Davis Cup team mate Janko Tipsarevic, who beat former world number one Juan Carlos Ferrero of Spain 7-5 6-7 7-5 6-2 in a slugfest that lasted almost four hours.
"It means there's going to be at least one Serbian in the semi-finals, which is great for our country," Djokovic said.
"We are great friends. It's not going to be easy to play him, but look, it's the quarter-finals and we both want to win."
Wozniacki has faced increasing criticism about her status as a world number one without a major title to her name but she showed all the predatory instincts of a prize fighter as she came back against Kuznetsova.
Seemingly down for the count after losing the first set in a tiebreaker then falling behind 4-1 in the second, she won 12 of the last 14 games and looked as fit at the end as she did at the start.
"I could have played another two or three sets if I had to," she said.
Williams's ability to overcome adversity has helped make her the finest player of her generation, and among the best of all time, but when doctors found a life-threatening blood clot on her lungs in March, winning the US Open was the last thing on the American's mind.
Yet here she is, riding high on emotion and with momentum building behind her.
Ivanovic, a former French Open champion, had loomed as a dangerous opponent, but Williams disposed of her with the ruthless determination of a woman on a mission.
"I think I'd like to say it's a bigger mountain like Everest ...(but) I don't ever want to get to the top of the mountain. I want to keep being able to reach something," she said.
Williams went into the match having won her two previous matches with Ivanovic, but Mother Nature was against her this time.
The wind was blowing hard at Flushing Meadows, making serving difficult. The gusts should have reduced the effectiveness of one of Williams's biggest weapons, but the 29-year-old handled the conditions better than her opponent.
"As I said even before the match, I think she's the favorite for the tournament," Ivanovic said.
"She's been playing really well, and it's gonna be tough (for anyone to beat her)."
Williams's next opponent is Russia's Anastasia Pavlyuchenkova, who avenged her agonising loss to Italy's Francesca Schiavone at this year's French Open to win 5-7 6-3 6-4.
It was sweet revenge for Pavlyuchenkova after she blew her chance of a first grand slam semi-finals appearance when she lost to Schiavone in the quarters at Roland Garros after leading 6-1 4-1.
"Of course I was thinking about it," Pavlyuchenkova said. "But it made me stronger I think. I never try to find excuses when I lose so I always blame myself."
The floodlit evening session was delayed by more than an hour and a half after 11th-seeded Frenchman Jo-Wilfried Tsonga needed almost four hours to beat American eighth seed Mardy Fish 6-4 6-7 3-6 6-4 6-2 in an enthralling center court clash.
Tsonga's reward is a quarter-final on Wednesday against either Roger Federer or Juan Monaco of Argentina, who had to wait until almost midnight to start their match.