World number nine Caroline Wozniacki was beaten in straight sets by world number 73 Garbine Muguruza, crashing out of the second round of the Miami Masters.
Wozniacki was taken to three sets last time out by Karolina Pliskova and she again made a poor start as Muguruza eased to the first set 6-2.
The Spaniard then took a 2-0 lead before the former world number one reeled off three straight games.
Both women held before Muguruza broke before serving out for a 6-2 6-4 win.
World number four Agnieszka Radwanska was taken to three sets by Magdalena Rybarikova before prevailing 7-6 (7/5) 2-6 6-3.
Radwanska edged the first set on a tie break only for her opponent to storm through the second set and force a decider.
Radwanska, who already has two titles to her name in 2013, then broke in the seventh game for a 4-3 lead and another break ensured victory.
World number five Li Na had no such problems, despatching Varvara Lepchenko 6-2 6-4 while Ajla Tomljanovic beat Andrea Petkovic in a marathon match, 0-6 6-4 7-6 (7/1).
Laura Robson passed up the chance to become British number one on Friday night, beaten in three sets in the second round of the WTA Sony Open by Alize Cornet.
The 19-year-old started well but, in a match which lasted two hours 36 minutes but was stretched over seven hours due to rain delays and power cuts in Miami, the 32nd-seeded Frenchwoman eventually emerged a 5-7 7-5 6-1 winner.
World number 36 Cornet, who wrapped up the win with a run of five straight games, will face American Lauren Davis in round three.