Sara Errani wagged her finger and cupped her hand to her ear for the U.S. Open fans who tried to will Venus Williams to victory.
Two points from defeat, the diminutive Italian rallied to force a third-set tiebreaker and outlasted Williams in a match that was both lopsided and tight.
Errani won 6-0, 0-6, 7-6 (5) in the third round Friday after Williams had a chance to serve out the match in the final set.
Williams twice came back from down a break in the third but was done in by too many unforced errors — she had 52 in the match.
Errani, seeded 13th, next faces 121st-ranked qualifier Mirjana Lucic-Baroni, who stunned second-seeded Simona Halep 7-6 (6), 6-2. The 32-year-old Lucic-Baroni hadn't made a Grand Slam round of 16 since 1999.
Trying to close out the match at 5-3 in the third set, the 19th-seeded Williams had one of her eight double-faults.
Errani won just 12 games in three previous meetings with Williams. But in the first set, Williams couldn't take advantage of her power, making just 30 per cent of her first serves.
Errani used her quickness to run Williams around the court, and the 34-year-old looked sluggish after playing a long doubles match with her sister on Thursday night.
"Even if I won the first set 6-love, she's an amazing player," Errani said in an on-court interview.
And in the second, Williams indeed looked like the seven-time major champion she is, pouncing on Errani's soft serves.
Errani just had a little more left in the final set.
"I was ready to just fight," said Errani, who wasn't thrilled with fans yelling out support for her opponent during her serves.
Williams had four more unforced errors in the tiebreaker. And Errani's quick reflexes, honed in doubles, gave her match point when she stabbed a backhand volley to return what looked like a sure winner by Williams.
The 5-foot-4 Errani reached the final at the French Open and the semis at the U.S. Open in 2012, but both times she was routed by power players: Maria Sharapova and Serena Williams.
Big sister Venus couldn't do the same Friday, though she had another chance at victory in a doubles match later with Serena.
Earlier, Peng Shuai again beat a seeded player to reach her second straight Grand Slam round of 16. Two days after upsetting fourth-seeded Agnieszka Radwanska, Peng defeated 28th-seeded Roberta Vinci 6-4, 6-3.
The Chinese veteran had gone two years without making it past the second round at a major tournament before her Wimbledon run this summer. But she didn't meet any seeded players in the first three rounds there before losing to eventual champion Petra Kvitova in straight sets.
This time, Peng, ranked 39th, will face 14th-seeded Lucie Safarova or 22nd-seeded Alize Cornet to try to reach her first Grand Slam quarterfinal.
Peng said she'll need "more courage" in that match.
She'll have the momentum of two straight sharp performances. With third-ranked Li Na sidelined by a knee injury, Peng's success is giving the fans in China something to cheer.
"I don't know how it's on Sunday," she said. "I hope I will be same."
The 31-year-old Vinci had made the quarterfinals at the U.S. Open the last two years but lost in the opening round at the first three major tournaments of 2014.
Ninth-seeded Jelena Jankovic advanced by routing Johanna Larsson 6-1, 6-0. The 96th-ranked Larsson had upset Sloane Stephens in the second round.
Sixth-seeded Angelique Kerber also was upset, losing 6-1, 7-5 to Belinda Bencic, a 17-year-old making her U.S. Open debut.
On the men's side, fourth-seeded David Ferrer advanced to the third round when Bernard Tomic withdrew because of a left hip injury.
Seventh-seeded Grigor Dimitrov and 20th-seeded Gael Monfils both moved on in straight sets.