Zheng Saisai upsets Olympic champion Monica Puig at U.S. Open

Zheng Saisai pulled off a stunning upset of Olympic champion and 32nd-seeded Monica Puig on Monday at the U.S. Open, just two weeks after Puig became the first gold medalist in Puerto Rico's history.

Zheng, a 22-year-old from China, defeated Puig in straight sets, 6-4, 6-2. in their first round Open matchup at Flushing Meadows in New York.

The 35th-ranked Puig jumped ahead 4-1 early, but Zheng battled back to even the set 4-4 and cruised from there. Puig had 28 unforced errors opposite Zheng's 13.

Zheng will face 59th-ranked Kateryna Bondarenko in the second round, looking to advance to the third round at a Grand Slam event for the first time in her career.

Meanwhile, Angelique Kerber -- who Puig beat in the gold-medal match at the 2016 Summer Olympics in Rio de Janerio, Brazil -- didn't have to work too hard to advance to the second round of the Open.

The second-seeded German led 6-0, 1-0 before her opponent, Slovenian Polona Hercog, retired early in the second round in the midst of 90-degree temperature. Hercog complained of dizziness and cramping before bowing out of the match after just 33 minutes.

Kerber, the reigning Australian Open winner, has a chance to overtake Venus Williams for the No. 1 ranking depending on her result at the U.S. Open.

Roberta Vinci, who bounced Williams in last year's U.S. Open en route to an appearance in the final, got off on the right foot on Monday. The seventh-seeded Italian breezed to a 6-2, 6-4 victory over German Anna-Lena Friedsam.

Vinci's countrywoman was not as fortunate as Francesca Schiavone was drilled by Svetlana Kuznetsova. The ninth-seeded Russian cruised to a 6-1, 6-2 victory.

In other completed matches on Monday, 14th-seeded Petra Kvitova of the Czech Republic posted a 7-5, 6-3 triumph over Jelena Ostapenko of Latvia and 24th-seeded Belinda Bencic of Switzerland outlasted American Samantha Crawford, 6-7 (6-8), 6-3, 6-4.

Seeded players who were bounced in the first round were Italian Sara Errani and Misaki Doi of Japan.

Errani, who is seeded 27th, dropped a 6-4, 7-6 (7-3) decision to American Shelby Rogers while Doi was upended by German Carina Witthoeft, who posted a 6-4, 6-1 triumph.