Britain's Cameron Norrie was beaten by Taylor Fritz in the Miami Open third round as top seed Daniil Medvedev overcame leg cramps to advance.
World number 56 Norrie forced a decider against American 22nd seed Fritz but ultimately fell 6-4 3-6 6-1.
Russia's Medvedev had cruised to a set and a 5-2 lead against Alexei Popyrin before he began to cramp in the heat.
Despite struggling to walk, Medvedev was able to secure a 7-6 (7-3) 6-7 (7-9) 6-4 win over the Australian.
"It's probably one of the sweetest victories in my career, because I was cramping like hell in the third set," Medvedev said.
"The only thing I was thinking about was not to fall down because if I fell down, I don't think I'd be able to get up."
Medvedev, who reached the Australian Open final in January, will face Frances Tiafoe of America for a place in the quarter-finals.
In the men's doubles, Britons Dan Evans and Neal Skupski fought back to beat fifth seeds Jamie Murray and Bruno Soares.
Evans and Skupski beat the British-Brazilian pair 5-7 7-6 (7-4) 10-7 to reach the last eight.
Bianca Andreescu continued her return from injury with a 7-6 (7-4) 6-7 (2-7) 6-4 victory over Amanda Anisimova of America.
The Canadian, who missed the entire 2020 season with a knee injury, will play in-form Garbine Muguruza of Spain in the round of 16.
Japanese second seed Naomi Osaka, who is in contention for the world number one ranking, also advanced when opponent Nina Stojanovic of Serbia withdrew with a right thigh injury.
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