Defending champion Naomi Osaka advanced to the US Open third round after Serbian qualifier Olga Danilovic withdrew ahead of their second-round match on Wednesday due to illness, tournament organisers announced.
The Japanese third seed and world number 145 Danilovic, who was competing in her first US Open main draw, were scheduled to be the day's second match in Arthur Ashe Stadium.
"I am so sad to have to (withdraw) from my match this morning," Danilovic wrote on social media.
"I have been feeling unwell these past few days dealing with a non-Covid related viral illness. I was really looking forward to playing against Naomi on Arthur Ashe Stadium today, but not to be this time."
Osaka, who beat Marie Bouzkova in the first round on Tuesday in her first grand slam appearance since she pulled out of the French Open in May, will face Canada's Leylah Fernandez who beat Estonian Kaia Kanepi 7-5 7-5.
Simona Halep shrugged off her recent injury anguish to advance to the third round for the first time in five years with a straight-sets win over Kristina Kucova.
The former world number one and two-time grand slam champion had been forced to miss this year's French Open and Wimbledon with a calf injury and a series of other concerns.
But despite playing with heavy strapping on her thigh, Halep was in dominant form as she swept aside the Slovakian lucky loser 6-3 6-1.
Halep said in her on-court interview: "I have to admit I am more confident, because before the tournament I was a little bit worried having not played for five or six months.
"It's always tough to handle the pressure but I think I did well. It's great to be back at the US Open and to play and win some matches."
Second seed Aryna Sabalenka notched her 40th win of the season with a 6-3 6-1 victory over Slovenia's Tamara Zidansek. The Belarusian took a hard fall in the opening set and kept icing her right hand and wrist during changeovers.
Earlier, 25th seed Daria Kasatkina rallied from a set down to beat Olympic silver medallist Marketa Vondrousova 3-6 6-4 6-4.
Czech eight seed Barbora Krejcikova sealed a dominant 6-3 6-1 victory over American Christina McHale after the match was moved to Louis Armstrong stadium following a rain delay.
Ninth seed and two-time Grand Slam winner Garbine Muguruza powered past Andrea Petkovic 6-4 6-2 to reach join them in the third round and claimed a first career win over the German in the process.
Teenager Coco Gauff was beaten in the all-American clash against Sloane Stephens.
Stephens, the 2017 champion who has slipped to 66 in the rankings, ran out a comprehensive 6-4 6-2 winner against the 21st seed.
Stephens, who has known Gauff since she was a child, said. "I love Coco, everyone knows that and I said that to her the end of the match.
"I feel so lucky to have seen her grow up since she was eight years old and I'm really proud of her."
