Two-time U.S. Open runner-up Caroline Wozniacki rallied from a set down for her first Grand Slam match win of 2016.

Wozniacki beat American qualifier Taylor Townsend 4-6, 6-3, 6-4 on Monday in the first-ever match at the new 8,000-seat Grandstand. She lost in the first round at this year's Australian Open and Wimbledon and skipped the French Open because of a right ankle injury that forced her to miss 2 1/2 months. The former No. 1 player's ranking has slipped to 74th, and she's unseeded at Flushing Meadows for the first time since her debut in 2007.

The 20-year-old Townsend, ranked 146th, was playing her second U.S. Open.

Australian Open champ Angelique Kerber also advanced to the second round at the U.S. Open when her opponent retired early in the second set without winning a game.

The second-seeded Kerber led 6-0, 1-0 after 33 minutes when Polona Hercog stopped. The 120th-ranked Slovenian won just nine points in seven games.

After the first set on a 90-degree day, Hercog took a medical timeout. Trainers checked her blood pressure and rubbed ice bags on her legs.

Hercog, who's never been past the third round at a major, came in 2-1 against Kerber in tour-level matches, though they hadn't met since 2011.

Kerber, who lost to Serena Williams in the Wimbledon final, has a chance to overtake her for the No. 1 ranking depending on their results at Flushing Meadows.

Olympic gold medallist Monica Puig has lost in her first-round match.

The 32nd-seeded Puig was upset by 61st-ranked Zheng Saisai 6-4, 6-2 on Monday. Puig had insisted Saturday that she was mentally ready for the year's final major after the whirlwind of the past couple of weeks, when she stunningly beat three Grand Slam champions to become Puerto Rico's first gold medallist and was feted at a parade on the island.

Still, she recognized it was quite possible her U.S. Open trip wouldn't go well, reminding herself that she's just 22 years old. She's now 1-3 at Flushing Meadows.

Zheng, also 22, upset Agnieszka Radwanska at the Olympics. She'll be seeking to make it to the third round at a major for the first time.

French Open champ Garbine Muguruza says she thinks the heat caused her to have trouble breathing early in her U.S Open first-round match.

The third-seeded Spaniard had to call for the trainer after dropping the first set to Belgian qualifier Elise Mertens with temperatures in the low 90s. Muguruza said after Monday's three-set victory that she had forgotten there were towels with ice available to cool off during changeovers. She also was able to relax more, which helped.

Muguruza says she had some trouble breathing in practice a couple of days earlier as well, but she felt fine during her warmup Monday. The conditions reminded her of the Australian Open, though she never remembered it being this uncomfortable in New York.

Then again, the 22-year-old has never stuck around past the second round in singles at the U.S. Open.

American teen CiCi Bellis is now 2 for 2 in the first round of the U.S. Open.

The darling of the 2014 tournament after she stunned Australian Open runner-up Dominika Cibulkova as a 15-year-old, Bellis is back in the main draw this year and beat 65th-ranked Viktorija Golubic 6-2, 6-3 on Monday.

Bellis, now 17, is ranked 158th and advanced through qualifying to make the field. She next faces another American, Shelby Rogers, who upset 27th-seeded Sara Errani.