Serena Williams survived her first test of the US Open to reach the quarter-finals in New York for the 15th time.
After winning the opening set against Kaia Kanepi in just 18 minutes and for the loss of six points, Williams found herself taken to a deciding set for the first time this tournament.
But she responded impressively to win 6-0 4-6 6-3 and set up a last-eight clash with eighth seed Karolina Pliskova, the last player to beat her at Flushing Meadows in the semi-finals two years ago.
Williams said: "It wasn't an easy match at all. She obviously knows how to play, she's had a lot of big wins. We always have tough matches. I was just happy to get through it to be honest."
Asked about the improvements she has made since losing in the final of Wimbledon to Angelique Kerber, Williams said: "The thing I've improved most is my fitness. Coming back from a baby is hard, harder than I thought."
After producing the most impressive display since her comeback to defeat sister Venus for the loss of just three games on Friday, it looked like Williams would be even more dominant against Kanepi, who upset top seed Simona Halep in round one.
Kanepi, who had never previously taken a set off her opponent, was simply steam-rollered but she dug in with a break of the Williams serve at the start of the second set and settled into the match.
She held onto her advantage until 4-2 and then broke Williams again with a brilliant game. A forehand cross-court winner drew applause from the 23-time grand slam singles champion before Kanepi left her opponent helpless again with a forehand down the line.
Williams made her fight all the way to take the set, retrieving one of the breaks and threatening to make it 5-5. But Kanepi produced two big serves to save break points and then clinched it on her second set point.
Williams knew how big the start of the third set was and she roared as she repelled Kanepi's threat to hold serve. It was a crucial moment and the 36-year-old quickly gained an advantage that she never threatened to lose.
Williams finished with a tally of 47 winners, including 18 aces.
