Maria Sharapova is out of the Miami Open after suffering a surprise second-round straight-sets defeat by world number 97 Daria Gavrilova.
World number two Sharapova, 27, was beaten 7-6 (7-4) 6-3 by 21-year-old fellow Russian Gavrilova.
There were no problems for fourth seed Caroline Wozniacki, who sealed a comfortable 6-0 6-1 win over American Madison Brengle in less than an hour.
In the men's tournament, Canada's Vasek Pospisil beat Juan Martin Del Potro.
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 kept him out since January.
Friday's play features British number ones Andy Murray, who faces American Donald Young, and Heather Watson, who plays Germany's Angelique Kerber.
World number three Rafael Nadal takes on fellow Spaniard Nicolas Almagro, while women's world number one Serena Williams plays Romanian Monica Niculescu.