Emma Radacanu crushed McCartney Kessler to reach Miami Open round of 16, leading 6-1, 3-0 after two sets, before the American opted to retire.
Radacanu had won eight straight games before Kessler exited the match following a chat with the physio.
The Brit will face the winner of Mirra Andreeva versus Amanda Anisimova on Monday in the last-16.
Raducanu backed up her stunning victory over Emma Navarro - her first against a top-10 player in her career - to set up her last-16 meeting.
Raducanu said: 'It's never a way any athlete wants to win a match. I wish McCartney a speedy recovery.
'She's in such good form right now. Playing so well. Playing so aggressive. We both had long last matches and, yeah, it's a tough physical sport. I wish her all the best.'