The year's final major got underway as play began at the U.S. Open with the bottom half of the women's draw seeing action.
Among the winners were second seed Aryna Sabalenka, third seed Naomi Osaka, fifth seed Elina Svitolina and eighth seed Barbora Krejcikova.
Top seeds move through
Osaka faced a challenge from Marie Bouzkova early on, but she eventually pulled away, winning eight of the last nine games to advance with a 6-4, 6-1 victory.
The defending champion hit 34 winners and saved all three break points she faced in the first set. She broke the feisty Czech to win the first set and never looked back in the 93-minute match.
"I actually thought I was doing pretty well on the return games", she said. "There were a couple of games that were pretty close, so I just kept telling myself to keep it really tight, hopefully she'll get nervous.
"My serve's definitely one of my biggest weapons. It's able to come in when it's really necessary. Hopefully it won't be that needed in the future."
Svitolina needed just over an hour to move on with an efficient 6-2, 6-3 win over qualifier Rebecca Marino. The Ukrainian broke the Canadian, back in the main draw here for the first time in a decade, four times en route to the win.
Krejcikova made her main draw debut here a rousing success, needing an hour, 14 minutes to dispatch Astra Sharma 6-0, 6-4. The Roland Garros champion won the first eight games of the match and saved both break points she faced.
While Osaka, Svitolina and Krejcikova advanced comfortably, Sabalenka was made to work for her victory, defeating Nina Stojanovic 6-4, 6-7 (4), 6-0.
After racing out to a 4-1 first set lead, the Serb rallied to level the set, but was broken at love two games later. A 4-2 second-set lead was gone after Stojanovic saved a match point and took the tiebreaker.
Cleaning up her game, the second seed blanked the Serb in the deciding set, finishing with 43 winners and 13 aces after two hours, 24 minutes of play.
Stephens edges Keys in 2017 final rematch
Sloane Stephens moved on after besting good friend Madison Keys 6-3, 1-6, 7-6 (7).
An early break for Stephens was retained throughout the opening set and she claimed a second break to take it by the same score she had won the first set of their final four years ago.
However, unlike that final, Keys stormed back in the second as she raced to a 4-0 lead in that set, then eased through to force a third set. Keys had 11 winners to just three unforced errors in the second set.
In the final set, Stephens held two break points at 4-4, but Keys used her power game to hold for 5-4. At 5-5, Stephens converted her fourth break point of the game to serve for the match, but Keys struck back immediately to force a tiebreaker.
In the breaker, the players jockeyed back and forth and Stephens saw two match points at 6-5 and 7-6 slip away. But Stephens reached a third match point after a netted forehand by Keys, and she took that chance after a Keys backhand flew wide.
Other seeded results
Ninth seed Garbine Muguruza needed two tiebreakers to oust Donna Vekic 7-6 (4), 7-6 (5). The Spaniard hit 29 winners in the two hour, 18 minute contest.
"It was a very tough first round", she told the media afterwards. "Having to play [Vekic] in the first round was very difficult, and I could see it on court.
"I knew it was two close sets that I was happy to close in the tiebreak, which can go any way at that point."
12th seed Simona Halep returned from an injury layoff to take out the dangerous Camila Giorgi 6-4, 7-6 (3).
15th seed Elise Mertens saved six match points before finally overcoming Rebecca Peterson 3-6, 7-6 (5), 7-6 (5). 16th seed and 2016 champion Angelique Kerber stormed from behind to eliminate Dayana Yastremska 3-6, 6-4, 7-6 (3).
18th seed and last year's runner-up Victoria Azarenka won the final 11 games of her match against Tereza Martincova to post a 6-4, 6-0 victory while 19th seed Elena Rybakina had no problems with Aliaksandra Sasnovich 6-3, 6-2.
20th seed Ons Jabeur also moved on with a 7-5, 7-5 victory over Alize Cornet while 21st seed Cori Gauff rallied from a set down to knock out Magda Linette 5-7, 6-3, 6-4.
25th seed Daria Kasatkina routed 2020 quarterfinalist Tsvetana Pironkova 6-2, 6-1 and 32nd seed Ekaterina Alexandrova cruised past Sara Errani 6-3, 6-2.
Joining Gauff and Stephens in the second round was 26th seed Danielle Collins, who defeated Carla Suarez Navarro 6-2, 6-4 in the Spaniard's final major match and Christina McHale, who got by fellow American Emma Navarro 6-1, 7-6 (5).
Eliminated were Bernarda Pera, ousted by Tamara Zidansek 6-4, 7-6 (4), Ann Li, knocked out by lucky loser Kristina Kucova 7-5, 6-1 and wild card Alycia Parks, losing to qualifier Olga Danilovic 6-3, 7-5.
Olympic sliver medalist Marketa Vondrousova was victorious against Elena-Gabriela Ruse 7-5, 6-0, Leylah Fernandez advanced past qualifier Ana Konjuh 7-6 (3), 6-2, Jasmine Paolini got by Yaroslava Shvedova 6-3, 6-2 and Andrea Petkovic beat Irina-Camelia Begu 6-2, 7-6 (3).
Maria Camila Osorio Serrano, the 2019 champion, blasted Ivana Jorovic 6-2, 6-2, qualifier Valentini Grammatikopoulou bested Anna Blinkova 6-3, 6-2 and Kaja Juvan crushed Heather Watson 6-1, 6-4.
There were three-set wins for qualifier Rebeka Masarova, who edged Ana Bogdan 6-7 (9), 6-2, 7-6 (9), Caroline Garcia, who came from a set down to knock out qualifier Harriet Dart 6-7 (6), 6-4, 6-2, Anhelina Kalinina, who rallied past lucky loser Mayar Sherif 4-6, 6-1, 6-1 and lucky loser Kamilla Rakhimova, a 2-6, 6-2, 6-3 winner over Kristina Mladenovic.