The US Open women's draw was blown wide open Friday as world number two Simona Halep, sixth seed Angelique Kerber and two-time champion Venus Williams tumbled out of the third round.
Two players at opposite ends of the tennis age spectrum authored the upsets of the day.
Croatian veteran Mirjana Lucic-Baroni, a former phenom now a 32-year-old veteran who had to fight through qualifying, shocked second-seeded Halep, the French Open runner-up 7-6 (8/6), 6-2.
And 17-year-old Swiss Belinda Bencic toppled Germany's Kerber 6-1, 7-5.
The reward for the 121st-ranked Lucic-Baroni is a fourth-round meeting with 13th-seeded Italian Sara Errani, who thwarted Williams 6-0, 0-6, 7-6 (7/5).
Bencic, the 2013 Wimbledon and French Open junior champion, now ranked 58th in the world, earned a meeting with former world number one Jelena Jankovic for a place in the quarter-finals.
"It's incredible, amazing, I feel goofy," said Lucic-Baroni, who reached a Grand Slam fourth round for the first time since making it to the 1999 Wimbledon semi-finals as a 17-year-old.
"After so many years, it's incredible. I live for this," she said. "I am 32. Every painful moment has been worth it."
The last Grand Slam of the year has now lost four of it's top 10 women's seeds, after the second-round departures of Agnieszka Radwanska and Ana Ivanovic.
Former champion and fifth seed Maria Sharapova was trying to buck the upset trend later Thursday when she took on big-hitting German Sabine Lisicki under the floodlights on Arthur Ashe Stadium court.
Errani's triumph over 19th-seeded Williams wasn't strictly speaking an upset, but the 13th-seeded Italian had to dig deep to notch her first career victory over the American after three defeats.
"I know that was a really tough match, even if I won the first set 6-0 she's an amazing player," Errani said.
And as she expected, five-time Grand Slam champion Williams battled back, racing through the second set to set up a third that turned into a slugfest.
Williams twice came back from a break down in the third, then broke Errani to serve for the match at 5-3.
She couldn't hang on, however, surrendering her serve as they battled to the tiebreaker, in which Errani took a 5-2 lead only for Williams to battle back to 5-5 before at last succumbing.
- Peng moves on -
China's Peng Shuai, showing no sign of a let-down after her second-round win over Radwanska, dispatched another seeded player in number 28 Robert Vinci 6-4, 6-3.
Peng managed to keep her nerves in check to equal her best US Open showing, a a fourth-round appearance in 2011.
"I have a lot of motivation," said Peng, who knows she has the eyes of China on her with compatriot Li Na sidelined by injury. "I wanted to fight and I really enjoyed the play."
The highest men's seed in action, number two Roger Federer, was to resume his quest for a sixth US Open title against big-serving Australian Sam Groth on Friday night.
The 104th-ranked Groth is averaging more than 16 aces per match this season, but in his first match against a top 10 player he'll be walking into the lion's den.
Federer boasts a 23-1 record in night matches on Arthur Ashe Stadium court
Ninth-seeded Jankovic, whose only Grand Slam final appearance was a runner-up finish at Flushing Meadows in 2008, was never in danger in a 6-1, 6-0 victory over Sweden's Johanna Larsson.
The surprises on the women's side overshadowed men's second-round action.
Fourth-seeded David Ferrer reached the third round without lifting his racquet when unpredictable Aussie Bernard Tomic withdrew with a hip injury.
Wimbledon semi-finalist Grigor Dimitrov of Bulgaria continued his best-ever US Open with a 6-1, 6-2, 6-2 victory over Dudi Sela, improving his career record against the Israeli to 3-0.
Croatia's 14th-seeded Marin Cilic, 18th seed Kevin Anderson of South Africa, 19th seed Feliciano Lopez and 20th seed Gael Monfils all advanced on cue.