Collins, a Florida native, reached her first Grand Slam final in January when she fell to Ashleigh Barty for the Australian Open title. She made early exits at the French Open and Wimbledon before reaching the fourth round at the US Open.

On Monday, Collins overcame a rocky serving performance that saw her put just 48 percent of her first serves in play while logging more double faults (seven) than aces (five). Her next opponent will be Italy's Martina Trevisan, who defeated Colombia's Camila Osorio 6-3, 6-4.

Three other US players won first-round matches: Madison Keys, Coco Vandeweghe and qualifier Louisa Chirico. Canada's Bianca Andreescu topped Russian lucky loser Liudmila Samsonova 7-6 (1), 4-6, 6-2 in the last match of the night.

Transylvania Open

Germany's Jule Niemeier and Italy's Jasmine Paolini posted upset wins in the first round at Cluj-Napoca, Romania.

Niemeier got past third-seeded Ana Bogdan of Romania 7-6 (5), 6-2, and Paolini knocked out sixth-seeded Marta Kostyuk of Ukraine 5-7, 6-3, 6-4.

Niemeier had eight double faults to Bogdan's one, but she came through in key moments, converting five of her seven break points. Paolini fell behind by two service breaks in the third set but won five of the last six games to pull away.

Also advancing to the second round were eighth-seeded Anna Bondar of Hungary, Dayana Yastremska of Ukraine, Anna Blinkova of Russia and Ysaline Bonaventure of Belgium.