Bethanie Mattek-Sands picked up her first top-10 victory since 2015, upsetting ninth-seeded Elina Svitolina 7-5, 6-4 on Thursday to reach the Miami Open's third round.
Mattek-Sands, who turned 32 on Thursday, is ranked only 158th in singles and needed a wild-card invitation to get into the hard-court tournament. She is ranked No. 1 in doubles.
Against Svitolina, Mattek-Sands saved 12 of the 15 break points she faced.
The woman with whom Mattek-Sands won the doubles championships at the past two Grand Slam tournaments, Lucie Safarova, eliminated 23rd-seeded Daria Gavrilova 6-2, 6-2 on Thursday. Safarova, the 2015 French Open runner-up, is ranked 36th, so was just outside the seedings at the hard-court tournament. She broke Gavrilova five times while losing serve only once herself.
In a couple of other surprises, 18th-seeded CoCo Vandeweghe was knocked out by 105th-ranked qualifier Jana Cepelova 6-2, 3-6, 6-1, and Kirsten Flipkens beat No. 29 Ana Konjuh 7-6 (4), 6-7 (6), 6-2.
Another surprise appeared to be developing when play was halted Thursday night because of rain: No. 6 Garbine Muguruza, the reigning French Open champion, was trailing 46th-ranked American Christina McHale 6-0, 3-2. They are scheduled to resume their second-round match Friday.
Second-seeded Karolina Pliskova, a finalist at last year's U.S. Open, had no trouble beating 86th-ranked Madison Brengle 6-1, 6-3 in about an hour to get to the third round.
Other seeded winners included No. 4 Dominika Cibulkova, No. 5 Agnieszka Radwanska, No. 12 Caroline Wozniacki, No. 17 Anastasia Pavlyuchenkova, No. 26 Mirjana Lucic-Baroni, No. 27 Yulia Putintseva and No. 30 Zhang Shuai.
In men's action, now things will get a little more challenging for Frances Tiafoe.
Tiafoe, a 19-year-old from Maryland, set up a second-round meeting against 18-time Grand Slam champion Roger Federer by beating Konstantin Kravchuk 7-5, 5-7, 6-1.
Tiafoe hit 12 aces and accumulated 14 break points, converting five, on the 32-year-old Kravchuk's serve. This was Tiafoe's third career match victory at a Grand Slam or Masters event. Kravchuk, meanwhile, dropped to 0-7 overall this season.
Tiafoe is ranked 101st, Kravchuk 103rd. The level of competition will rise against the fourth-seeded — and former No. 1-ranked — Federer.
In other men's first-round action, Horacio Zeballos came back to beat Gastao Elias 6-7 (2), 6-2, 7-5 and earn a matchup against top-seeded Stan Wawrinka next. Wawrinka lost to Federer in the final at Indian Wells, California, on Sunday.
Elsewhere, Adrian Mannarino defeated qualifier Benjamin Becker 7-6 (6), 6-3 and will next face No. 32 Paolo Lorenzi, Borna Coric edged Marcel Granollers 7-6 (6), 4-6, 6-3, and Tommy Robredo topped Nikoloz Basilashvili 3-6, 6-4, 6-2.