Serena Williams has eased past compatriot Vania King 6-1 6-0 to record her 80th US Open win but former world No.
1 Ana Ivanovic became the second top 10 seed to suffer a shock early exit.
World No.1 Williams, chasing a third successive New York title, her sixth in total and an 18th major, fired 25 winners past King on Thursday and broke serve six times, wrapping up victory on a windswept Arthur Ashe Stadium in just 56 minutes.
It was her second win over an American at the tournament this week after beating teenager Taylor Townsend and next she will face another in Varvara Lepchenko for a place in the last 16.
“It’s so hard to play in the wind but I am happy to get through a solid match with the conditions today,” said the top seed.
“It wasn’t easy, but you have to be able to adjust. I had fun out here, I enjoy playing out on Arthur Ashe court.”
Williams came into the US Open without a major this year but with five tour titles.
One of those came in Cincinnati where she defeated former world No.1 Ana Ivanovic whose Flushing Meadows campaign was ended by Karolina Pliskova of the Czech Republic.
Ivanovic, the eighth seed, lost 7-5 6-4 to the 42nd-ranked Pliskova who has reached the third round at a major for the first time.
The Serb followed fourth-seeded Agnieszka Radwanska out of the tournament after the Pole had been beaten by Peng Shuai 24 hours earlier.
Ivanovic came into New York as a potential title contender having captured trophies at Auckland, Monterrey and Birmingham this year while returning to the top ten for the first time in five years.
But the former French Open champion was undone on Thursday by 29 unforced errors while facing 13 break points as she crashed to her earliest exit in New York in six years.
“It’s very disappointing. It’s never easy to finish this early,” said Ivanovic, whose summer build-up had included a runners-up spot behind Serena Williams at Cincinnati.
“I’m definitely going to assess what went wrong and what I can work on. I really felt it wasn’t my game out there today.”
Ivanovic’s fellow former world No.1 Victoria Azarenka, the runner-up to Williams for the past two years, made it to third round, winning nine games in succession from 0-3 down to defeat Christine McHale of the United States 6-3 6-2.
“She started really well and was hitting her targets. I was on the back foot but eventually I got my momentum,” said Azarenka.