Ashleigh Barty got off to a hot start against Elina Svitolina but was ultimately beaten in straight sets in the round of 16 at the Miami Open.

The Australian 21st seed led 5-2 in the opening set but the Ukrainian fourth seed fought back to win 7-5 6-4 in a one hour and 20 minutes.

Svitolina defeated Barty’s compatriot Daria Gavrilova in the previous round.

US Open champion Sloane Stephens upset Spanish third seed Garbine Muguruza 6-3 6-4 to reach the quarter-finals while Czech Karolina Pliskova advanced when her opponent retired hurt.

Stephens, who raised her game to levels similar to last year when she captured a maiden major title in New York, dictated play against the Wimbledon champion from the start.

The American 13th seed raced out of the gate by taking eight of the first nine points of the match.

After securing a break in the fifth game of the second set, Stephens sent Muguruza packing with a backhand on the line on her second match point.

Stephens, who equalled her best result in Miami having also reached the quarter-finals in 2015, will next face German 10th seed Angelique Kerber, a 6-7 (7-1) 7-6 (7-5) 6-3 winner over Chinese qualifier Wang Yafan.

Fifth seed Pliskova, who reached last year’s Miami semis, was first to reach the quarters after Zarina Diyas of Kazakhstan was forced to retire injured while trailing 6-2 2-1.

Former world No.1 Pliskova was efficient from the start and did not drop a point on her first serves in the entire match against Diyas, who took multiple medical time-outs and was visibly struggling with her movement.