Maria Sharapova suffered a shock second round exit at the Miami Open as she lost in straight sets to world number 97 Daria Gavrilova.
Gavrilova had only moved into the world's top 100 this week but she gave a confident performance to overcome her fellow Russian 7-6 (7/4) 6-3.
The wildcard led 5-3 in the first set but lost three games in a row before taking the tie break and then closed out against the second seed after opening up a 3-0 lead in the second.
Fourth seed Caroline Wozniacki had a far easier time of things as she eased to a 6-0 6-1 win over Madison Brengle in less than an hour.
That may have been a disappointment for the home crowd but veteran Venus Williams was able to lift their spirits with a 6-3 6-2 win over Urszula Radwanska.
The Pole's elder sister and seventh seed Agnieszka progressed with a 6-4 7-5 win over Anna Schmiedlova while Ekaterina Makarova beat Karin Knapp 6-1 6-3 and Andrea Petkovic of Germany eased into the third round thanks to a 6-2 6-2 victory against Christina McHale.
France's Alize Cornet defeated Elena Vesnina 6-4 6-1 while Irina-Camelia Begu, Carla Suarez Navarro, Kristina Mladenovic, Kaia Kanepi, Paula Badosa Gibert, Karolina Pliskova, Samantha Stosur, Kurumi Nara and Elina Svitolina also progressed.
In the men's tournament, Canada's Vasek Pospisil saw off the challenge of Juan Martin Del Potro to book his place in the second round.
Pospisil claimed a 6-4 7-6 (9/7) win over the Argentinian, who was making his return after suffering a wrist injury that had seen him ruled out since January.
Pospisil will now face ninth seed Grigor Dimitrov in the next round.
Elsewhere in the first round, Mikhail Kukushkin defeated Steve Johnson 6-4 3-6 6-2 while Martin Klizan came through 6-3 6-4 against Paolo Lorenzi to line up a meeting with top seed and defending champion Novak Djokovic.
Italy's Simone Bolelli beat Marcos Baghdatis 6-2 6-2 while Mikhail Youzhny came from a set down to win 4-6 6-2 6-3 against Andrey Golubev.
Teymuraz Gabashvili will face fifth seed Milos Raonic next after fighting back for a 0-6 7-6 (7/4) 6-3 win over Pablo Andujar and Federico Delbonis' 7-6 (7/4) 3-6 7-5 win over Jiri Vesely earned him a meeting with David Ferrer.
Borna Coric, Alexander Zverev, Alejandro Falla, Jerzy Janowicz, Alexandr Dolgopolov, Jurgen Melzer, Andrey Rublev, Steve Darcis and Thomaz Bellucci also came through their first round matches.