Serena Williams rallied to defeat the 2017 winner Sloane Stephens in the third round of the US Open and move into the women's singles last 16 on Saturday.

Third seed Williams essayed a signature comeback and served 12 aces to edge past her American compatriot Stephens 2-6, 6-2, 6-2 in one hour and 43 minutes.

"It was intense I have to say. We always have some really incredible matches and it brings out the best of my fitness when I play Sloane. I tried to stay calm and be more serene after the loss of the first set. I knew it was important for me just to keep trying, just to keep going," Xinhua news agency quoted Williams as saying.

"Right now I'm really focused on the US Open, and nothing else I can think about right now," said Williams, who has won six of her 23 Grand Slam singles' titles at the US Open.

Stephens went up a double break to carve out a 5-2 lead and served out the opening set in just 38 minutes. But Williams began to raise her game and served much better in the second set, lifting her first-serve percentage from 50 to 76 per cent. By breaking twice, Williams won the second set to level the match.

Then in the third set, Williams was rarely troubled by her rival enroute to victory.

She will take on the 15th-seeded Maria Sakkari of Greece in the last 16, who stunned Williams at the Western and Southern Open last month.

"Serena is Serena," Sakkari said after advancing earlier on Saturday by beating 19-year-old American Amanda Anisimova 6-3, 6-1. "You have to come up with some great tennis. Otherwise there is no chance against her."