Julie Coin returns a shot to top-seeded Ana Ivanovic at the U.S. Open. Coin defeated Ivanovic, 6-3, 4-6, 6-3, on Thursday in New York.

New York ? Even for the mathematics major from Clemson, it just didn’t add up: How could someone who recently struggled so badly she wanted to quit tennis stay on the court with the No. 1 player in the world?

Ana Ivanovic probably wondered the same thing.

In one of the biggest upsets in the sport’s history, the top-seeded Ivanovic was ousted from the U.S. Open, stunned by 188th-ranked Julie Coin, 6-3, 4-6, 6-3, in the second round Thursday.

“Obviously, if you would ask me at the moment if I’m playing like a No. 1, probably not,” Ivanovic said. “It’s very frustrating because I know I can play so much better.”

Never before in the Open era that began in 1968 had the No. 1 woman lost this early in the tournament. Plus, all the favorites had been breezing at Flushing Meadows.

The French Open champion seemed to be rallying in the third set when Coin suddenly recovered to win 10 straight points. The 25-year-old French math whiz screamed when Ivanovic’s last shot sailed out, then hopped for joy and hit an extra ball high into the stands.

“I don’t know how I’m going to sleep tonight,” Coin said. “I don’t know when I will realize everything.”

Coin spent much of the year playing in minor-league events and nearly got knocked out of the qualifying event merely to make it into the Open. She recently played so poorly she thought about giving up the sport and relying on her degree.

Top-seeded Rafael Nadal avoided a similar fate on the men’s side, beating Ryler De Heart, 6-1, 6-2, 6-4.

Venus Williams and sister Serena won earlier in the day. James Blake, David Ferrer, Andy Murray and Dinara Safina joined them in moving into the third round. No. 14 Ivo Karlovic posted the stat of the day while beating Florent Serra in straight sets – he finished ahead in aces 42-0.

Next up for Coin in the third round is No. 32 Amelie Mauresmo, who beat Kaia Kanepi, 2-6, 6-4, 6-0.

Fourth-seeded Serena Williams routed Elena Vesnina, 6-1, 6-1 and seventh-seeded Venus Williams overwhelmed Rossana de Los Rios, 6-0, 6-3 in only 59 minutes.