Serena Williams lived up to her billing as US Open favourite by seeing off Victoria Azarenka to reach the last 16.
Williams, seeded 28th after injury, hit top form in the first set but Azarenka fought back brilliantly from a break down to force a second-set tie-break.
The fourth seed saved four match points before Williams won 6-1 7-6 (7-5) to set up a meeting with Ana Ivanovic, who beat Sloane Stephens 6-3 6-4.
Top seed Caroline Wozniacki advanced with a 6-2 6-4 win against Vania King.
But that match was nothing compared to Williams and Azarenka, a superb advert for women's tennis and an encounter which would have made a worthy final.
Williams got off to a blistering start, holding serve to 15 with a huge ace in game one before breaking twice for a 4-0 lead.
Azarenka registered only six points in the first four games and, after saving a break point at 5-1, Williams sealed the 28-minute first set with a perfectly-placed backhand up the line.
However, the second set was a far more even contest - Azarenka showing superb levels of determination and energy to stick with her big-hitting opponent.
The Belarusian saved two break points in game one and then deservedly registered one of her own in game four.
Williams was rescued by her serve and eventually made her move with a stunning crosscourt forehand to break for 4-3.
But Azarenka chased every ball to save three match points in game nine and then a fourth in game 10 en route to breaking back in dramatic fashion.
She led for the first time in the match with a gritty hold for 6-5, only for Williams to level and prove more consistent in the tie-break.
"I'm glad to be in second week," said the 13-time Grand Slam champion. "I'm pleased but I want to do better and I'm looking forward to my next match.
"I think in the first set I played some really good tennis. In the second, she kind of dictated. I probably could have played better."
It was a similar story for Wozniacki, who outclassed King in the opening set before seeing a break overturned as she stumbled through the second.
The Dane finished with stats of 14 winners and 17 unforced errors, and will next face either Akgul Amanmuradova or Svetlana Kuznetsova.
"Today I just tried to keep my feet going and if you do that it then you can get in the right position on court," said Wozniacki.
"I am playing well and I'm happy to be in the fourth round."
Seventh seed Francesca Schiavone saved a match point before turning round her match against Chanelle Scheepers of South Africa.
The Italian, who lifted the French Open in 2010, was staring at a straight-sets defeat but eventually came through 5-7 7-6 (7-5) 6-3.
She now plays 17th seed Anastasia Pavlyuchenkova of Russia, who shocked 11th seed Jelena Jankovic 6-4 6-4.
The unseeded Spaniard Carla Suarez Navarro advanced to the fourth round with a 6-0 6-4 win against compatriot Silvia Soler-Espinosa.
Suarez Navarro will play 10th seed Andrea Petkovic after the German beat 18th seed Roberta Vinci of Italy 6-4 6-0.
The win for Petkovic means Germany have three women in the fourth round of the US Open for the first time since 1987. Sabine Lisicki and Angelique Kerber have also made it through to the last sixteen.