Former world number one Ana Ivanovic became the second top 10 seed to suffer a shock early US Open exit when she was knocked out by Karolina Pliskova of the Czech Republic on Thursday.
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.
"I played a good match, the best of the year," said 22-year-old Pliskova whose twin sister Krystina also played the main draw this year.
"I didn't really have a strategy. I just played my game, focussed on myself and I did it," added the slender Czech who sports a pair of colourful tattoos on her left upper arm and left thigh.
Ivanovic's fellow former world number one Victoria Azarenka, the runner-up to Serena 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.
Also going through to the third round early on Thursday was veteran Italian Flavia Pennetta, one of 15 players aged 30 years or older in the draw, who was a semi-finalist in 2013.
On the men's side, 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).
Sam Querrey beat Spain's Guillermo Garcia Lopez for the fourth time in four meetings -- including last week's Winston Salem warm-up -- with his 6-3, 6-4, 6-4 victory assuring the host country of at least one man in the last 32.
Later Thursday, top seeds Serena Williams and Novak Djokovic target places in the third round.
Five-time champion and world number one Williams faces a second successive American opponent in 25-year-old Vania King where victory will give her the 80th win of her career at the tournament.
Williams has never lost before the third round with her earliest defeat being the last-32 loss she suffered on her 1998 debut.
Djokovic, the 2011 champion who has reached the last four finals, faces 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.