Johanna Konta's defence of her Miami Open crown came to an end at the hands of veteran American Venus Williams in three sets on Monday.
British No 1 Konta, seeded 11th at the event, lost the momentum after winning the opening set as 37-year-old Williams recorded a 5-7 6-1 6-2 victory in their last-16 clash.
Williams, who was forced to save three match points against Kiki Bertens on Sunday, remains on course to land her fourth title in Florida.
Sloane Stephens stunned world No 3 Garbine Muguruza as the seeds continued to tumble in Florida.
Muguruza had been the highest ranked player left in the women's draw following the early exits of world No 1 Simona Halep and Caroline Wozniacki in the earlier rounds.
But US Open champion Stephens dominated Spain's reigning Wimbledon champion to record a 6-3 6-4 win and advance to the quarter-finals where she will face Germany's Angelique Kerber for a place in the semi-finals.
Kerber was made to sweat in her gruelling clash with China's Wang Yafan before prevailing 6-7 (1-7) 7-6 (7-5) 6-3.
Earlier, Czech fifth seed Karolina Pliskova booked her place in the last eight after opponent Zarina Diyas retired in the second set.
Pliskova was leading 6-2 2-1 when the Kazakh player abandoned the match with an injury. Next up for Pliskova will be a meeting with former world No 1 Victoria Azarenka after she beat Polish 30th seed Agnieszka Radwanska 6-2 6-2.