New York - Serena Williams has eased past compatriot Vania King 6-1, 6-0 to record her 80th US Open win on Thursday but former world number one Ana Ivanovic became the second top 10 seed to suffer a shock early exit.
World number one Williams, chasing a third successive New York title, her sixth in total and an 18th major, fired 25 winners past King 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 whose colourful leopard print dress was as striking as her tennis.
"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 number one 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."- Azarenka on charge -
Ivanovic's fellow former world number one 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.
The United States only saw three men making the second round -- the country's lowest total in the history of the tournament -- but 13th seed John Isner and Sam Querrey have made it to the last 32.
Isner, a quarter-finalist in 2011, beat Germany's Jan-Lennard Struff 7-6 (7/5), 6-4, 6-2 while Querrey beat Spain's Guillermo Garcia Lopez for the fourth time in four meetings -- including last week's Winston Salem warm-up -- thanks to a 6-3, 6-4, 6-4 victory.
Japanese 10th seed Kei Nishikori, whose best run is a fourth round appearance in 2008, progressed when Spanish opponent Pablo Andujar retired at 6-4, 6-1 down.
French wildcard Michael Llodra was also forced to quit after dropping the first set against German 22nd seed Philipp Kohlschreiber.
So far there have been nine retirements at the US Open where daily temperatures have been touching 32 degrees (90F).
French ninth seed Jo-Wilfried Tsonga also progresses, 6-3, 6-4, 6-4 against Kazakhstan's Aleksandr Nedovyesov.
Later Thursday, men's top seed Novak Djokovic, the 2011 champion who has reached the last four finals, faced veteran French player Paul-Henri Mathieu whose only win in six meetings against the Serb came back in 2006.
Mathieu had a gruelling first round win, getting past Gilles Muller of Luxemburg where he overcame 38 aces and saved 12 break points.
Andy Murray, the 2012 champion, meets 27-year-old German qualifier Matthias Bachinger, the world number 235 who won his first career Grand Slam match against Radek Stepanek on Monday.
Results on Thursday from the fourth day of the US Open, the final Grand Slam tournament of 2014 (x denotes seeded player):
Sam Querrey (USA) bt Guillermo Garcia-Lopez (ESP x28) 6-3, 6-4, 6-4
Philipp Kohlschreiber (GER x22) bt Michael Llodra (FRA) 6-2 - retired
John Isner (USA x13) bt Jan-Lennard Struff (GER) 7-6 (7/5), 6-4, 6-2
Jo-Wilfried Tsonga (FRA) bt Aleksandr Nedovyesov (KAZ) 6-3, 6-4, 6-4
Pablo Carreno-Busta (ESP) bt Benoit Paire (FRA) 6-1, 6-4, 3-6, 6-3
Kei Nishikori (JPN x10) bt Pablo Andujar (ESP) 6-4, 6-1 - retired
Serena Williams (USA x1) bt Vania King (USA) 6-1, 6-0
Varvara Lepchenko (USA) bt Mona Barthel (GER) 6-4, 6-0
Flavia Pennetta (ITA x11) bt Shelby Rogers (USA) 6-4, 6-3
Casey Dellacqua (AUS x29) bt Wang Qiang (CHN) 4-6, 6-4, 6-2
Karolina Pliskova (CZE) bt Ana Ivanovic (SRB x8) 7-5, 6-4
Victoria Azarenka (BLR x16) bt Christina McHale (USA) 6-3, 6-2