Garbine Muguruza rallied from a set down to reach the second round of the US Open after Elise Mertens threatened to pull off a big upset, while Angelique Kerber had no such trouble on day one.

Muguruza came into the final major of the year with only one singles victory to her name at Flushing Meadows and the French Open champion was in danger of making a first-round exit before battling back to win 2-6 6-0 6-3.

Belgian Mertens was playing in the main draw of a grand slam singles tournament for the first time and gave a great account of herself on Louis Armstrong, but was unable to take advantage of an unconvincing display from the third seed from Spain.

Kerber spent just over half an hour on court before opponent Polona Hercog retired due to illness with the second seed in complete command leading 6-0 1-0 at Arthur Ashe Stadium, where the new roof was not required on a sunny Monday.

Last year's runner-up Roberta Vinci, who ending Serena Williams' quest to win a maiden calendar Grand Slam with a sensational semi-final victory in 2015, also coasted into the second round by ousting Anna-Lena Friedsam 6-2 6-4.

Former world number one Caroline Wozniacki came back from a set down to advance in the first contest on the new Grandstand, but there was disappointment for Coco Vandeweghe as she crashed out of her home major with a shock defeat to Naomi Osaka.

UNINSPIRING START FROM MUGURUZA

Muguruza was ousted in the second round of Wimbledon and suffered a drubbing at the hands of Monica Puig in the Rio Olympics this month before putting together a run to the semi-finals of the Western & Southern.

The world number three will have to raise her game if she is to have any chance of claiming a second grand slam victory in New York, but the manner in which she responded to losing the first set with a bagel in the second showed her strength of character.

Seven double faults and 40 unforced errors would be a concern for the powerful 22-year-old, who will come up against Anastasija Sevastova in the second round.