Former World No 1 Ana Ivanovic was upset 7-5, 6-4 by 42nd-ranked Karolina Pliskova of the Czech Republic in the second round of the US Open on Thursday.
Pliskova, 22, took advantage of a slew of miscues from the eighth-seeded Serb during a windy, 80-minute match in Louis Armstrong Stadium to reach the third round for the first time in a Grand Slam event.
Ivanovic, 26, double-faulted seven times and committed 29 unforced errors as she had trouble handling the heavy, flat groundstrokes from the Czech.
"It's definitely my best match this year," said Pliskova, who has yet to win on the WTA Tour but twice has been a runner-up this year.
"I didn't have that much strategy. I just went and played my game. I want to enjoy myself and I did it."
Ivanovic began the year's fourth and final Grand Slam full of confidence after a runner-up finish in the Cincinnati tune-up event.
"It was very tough out there for me today," said Ivanovic, who has wins over Serena Williams and Maria Sharapova this season in battling back into the world top 10.
"I really struggled to find my rhythm and made way too many unforced errors. It was just really a bad day."
Pliskova, the 2010 Australian Open girls' champion, has been making tour progress in singles after gaining attention for her success in doubles, partnering with twin sister Kristyna last year to become the first twins to ever win a WTA doubles title.
"My serve, that's the main point," said Pliskova, who had seven aces in subduing Ivanovic.
"I was serving well. But I was trying to play aggressive. I knew if I was not going to be that aggressive she was going to make more winners than I, so I was trying to stay aggressive and to go to every point.
"She did a few mistakes which I think was the deciding point."
Pliskova will next meet 29th seed Casey Dellacqua of Australia, who overcame China's Wang Qiang 4-6, 6-4, 6-2.
World number ones Novak Djokovic and Serena Williams sailed through brisk winds to dismiss over-matched opponents on Thursday and land comfortably in the third round.
Gusty breeze led Williams to misfire for three double faults in her first service game before the two-time defending champion found her bearings to cruise by fellow American Vania King 6-1, 6-0 in 56 minutes.
"It's so hard to play in the wind," said the top-seeded Williams, seeking her sixth U.S. Open title and 18th career Grand Slam singles crown. "I'm very happy to get through a solid match with the conditions today.
"You have to be able to adjust," added Williams, who powered through 25 winners to just five for King.
Wimbledon champion Djokovic followed Williams onto the Arthur Ashe Stadium court and was equally efficient, gliding past French veteran Paul-Henri Mathieu 6-1 6-3 6-0.
Djokovic sounded the same note as Williams after advancing.
"It was very windy but I was able to adjust. I used my serve efficiently, getting them in," said the Serb, who pounded in 75 percent of his first serves including 13 aces without a double fault.
Djokovic joked about how his coach, Boris Becker, whose big serve in his playing days brought him the nickname "Boom Boom," would not be impressed.
"Thirteen aces he used to make in one set," joked Djokovic.
"It's not a big number for him, but big for me. Glad it was working."
Eighth seed Andy Murray, who overcame cramping in his first-round victory, roared past unseeded German Matthias Bachinger in straight sets to also reach the third round.
Murray, the 2012 U.S. champion and 2013 Wimbledon winner, looked fresh and strong in cool, breezy conditions as he rolled to a 6-3, 6-3, 6-4 victory at Arthur Ashe Stadium in his first meeting against the German.
The Briton, never broken in the one hour 46-minute match, ripped 36 winners, more than doubling the 17 unforced errors he was charged with.
Murray said he was well over the effects of the cramps he suffered in Monday's first-round match against Dutchman Robin Haase.
"I felt fine the last couple days. Had no problems on the Tuesday or Wednesday," the Scotsman said.
"I practiced well and didn't have any problem. Tonight was fine, too."
Murray will next meet Russian Andrey Kuznetsov, who eliminated 31st seed Fernando Verdasco of Spain in five sets 6-3, 4-6, 4-6, 7-5, 6-3.
"He's had a couple big wins in the slams this year," said Murray.
"He beat (David) Ferrer at Wimbledon and obviously today against Verdasco.
"I've never played him before. I don't know his game that well, but I've seen him play a little bit. He hits the ball pretty flat.
"Likes to go for his shots a lot. This court's fairly quick, so that will probably help him."
Fifth seed Milos Raonic blasted his way into the third round of the U.S. Open, pounding 26 aces past Peter Gojowczyk on his way to a 7-6 (4), 5-7, 6-4, 7-6 (3) win.
Raonic, who became the first Canadian man to reach a Grand Slam semi-final at Wimbledon in July, produced a muscular display on a gusty evening at the US National Tennis Center.
Along with his 26 aces, Raonic fired 64 winners in the three hour scrap with the German qualifier.
Raonic will next meet Victor Estrella Burgos of the Dominican Republic who beat Croatia's Borna Coric 7-6 (2), 4-6, 6-4, 6-2.
Hard-hitting American Sam Querrey limited his time spent on court against a healthy Guillermo Garcia-Lopez with his racket, beating the 28th-seeded Spaniard 6-3, 6-4, 6-4.
"If I keep playing like I played today I will keep going in the right direction and hopefully get back up in the top 20," said Querrey, who has slipped to 57th in the rankings.
Querrey won the dubious prize of facing Djokovic in the next round.
"Sam is one of the top Americans," noted Djokovic. "He has a big serve on any surface. If he serves well, he is dangerous."
Most other leading seeds also stayed on course at the year's last Grand Slam.
Ninth seed Jo-Wilfried Tsonga of France breezed by Aleksandr Nedovyesov of Kazakhstan 6-3,6-4,6-4 and big-serving American John Isner, the 13th seed, beat German Jan-Lennard Struff in straight sets.
Tenth seed Kei Nishikori of Japan had a shorter trip through the second round as he advanced after Spain's Pablo Andujar retired while trailing 6-4,6-1.
Philipp Kohlschreiber, the 22nd seed, had an even quicker time booking a place in the third round as France's Michael Llodra retired after dropping the first set 6-2, due to an ailing elbow.
Those retirements raised the number of withdrawals in the men's draw to eight not yet halfway through the second round.
Also shown the door along with Ivanovic was 2011 champion Sam Stosur of Australia, who lost a third-set tiebreaker 10-8 to Kaia Kanepi of Estonia.
Wimbledon champion Petra Kvitova, the third seed, took her expected place in the third round with a 6-4, 6-2 victory over fellow Czech Petra Cetkovska.
Eleventh seed Flavia Pennetta of Italy and 16th-seeded Victoria Azarenka of Belarus also advanced -- both at the expense of American opponents.
Pennetta dismissed Shelby Rogers 6-4 6-3, while two-time Australian Open winner Azarenka roared back from a 3-love, 0-40 deficit at the start to beat Christina McHale 6-3 6-2.