The 32-year-old, ranked 158th in the world who won mixed double gold alongside Jack Sock at Rio 2016, produced a superb display on her way to a 7-5, 6-4 win to set up a meeting with Russia's Anastasia Pavlyuchenkova.

Unseeded Czech Lucie Šafářová is also through as she knocked out 23rd-seeded Daria Gavrilova of Australia by clinching a comfortable 6-2, 6-2 triumph.

It proved to be a difficult day in the American city for a number of the seeded players as Coco Vandeweghe, the 18th seed, was beaten by qualifier Jana Čepelová of Slovakia.

Čepelová came into the match as the outsider but upset the odds with a 6-2, 3-6, 6-1.

Belgium's Kirsten Flipkens also sprung a surprise as she ousted 29th seed Ana Konjuh of Croatia 7-6, 6-7, 6-2.

Fourth seed Dominika Cibulková of Slovakia awaits for Flipkens in round three after she dispatched Paraguay's Verónica Cepede Royg 6-3, 6-2.

French Open champion Garbiñe Muguruza of Spain is on the brink of a shock exit as she trailed American Christina McHale, the world number 46, 6-3, 3-2 when bad weather brought a premature end to the day's play.

Elsewhere in the women's draw, former world number one Caroline Wozniacki prolonged her stay at the tournament with victory over Varvara Lepchenko of the United States.

The Dane, who recently spoke out concerning Russia's Maria Sharapova imminent return to the tour, dropped just three games as she secured a 6-1, 6-2 win.

In the men's competition, unseeded American Frances Tiafoe booked a date with 18-time Grand Slam champion Roger Federer as he overcame Russia's Konstantin Kravchuk 7-5, 5-7, 6-1.

Argentina's Horacio Zeballos joined Tiafoe in the next round as he fought back from a set down to beat Gastao Elias of Portugal 6-7, 6-2, 7-5.

Zeballos will entertain top-seeded Stanislas Wawrinka of Switzerland in round two.

The event in Miami continues tomorrow.