Defending champion Jakub Mensik was bundled out of the Miami Open on Monday, losing a gruelling three-set battle against 19th seed Frances Tiafoe 7-6 (7/4), 4-6, 7-6 (13/11).
A 2hr 55min third-round slug-fest in searing late afternoon heat was decided by a marathon third set tiebreak when Czech 20-year-old Mensik saved six match points before finally succumbing at the Hard Rock Stadium.
US hope Tiafoe, who himself saved two match points in the final tiebreak, sealed victory after converting his seventh match point when a Mensik return of serve drifted wide.
Tiafoe will face France's Terence Atmane in the last 16. Atmane powered into the fourth round with a 6-3, 1-6, 6-3 upset of Canadian seventh seed Felix Auger-Aliassime on Monday.
Auger-Aliassime joined a lengthening list of seeds who have stumbled in the early rounds in Miami that includes Indian Wells runner-up Daniil Medvedev, who was shown the door earlier Monday in a 6-0, 4-6, 7-5 loss to Argentina's Francisco Cerundolo.
Medvedev looked to be heading for a brisk exit after 18th seed Cerundolo raced through the first set in just over 20 minutes.
But Medvedev rallied in the second to level the match, and there was little to choose between the two in the deciding set.
Cerundolo held to take a 6-5 lead and then scored the decisive break to clinch victory when Medvedev double-faulted on match point.
"It was a great match, super tough," Cerundolo said. "It was my first match against Daniil. He's probably one of the only guys on Tour who I have never played.
"I didn't know what to expect. He has had a great year so far, so I didn't expect to be up 6-0 in the first set and a break up in the second. He started playing super good. But I kept believing and stayed there in the match."
Medvedev's exit followed the departures of world No 1 Carlos Alcaraz, Australian fifth seed Alex De Minaur and eighth seed Ben Shelton.
Another French player also progressed in the men's draw on Monday, with 31st seed Ugo Humbert beating Kazakhstan's Alexander Shevchenko 6-4, 7-6 (7/2).
GAUFF, MUCHOVA ADVANCE
In the women's draw, American fourth seed Coco Gauff was given a scare by veteran Romanian Sorana Cirstea before advancing to the quarterfinals with a 6-4, 3-6, 6-2 victory.
Gauff looked to be progressing smoothly into the last eight after taking the first set and motoring into a 3-1 lead in the second.
But Cirstea pounced on a jittery spell from Gauff to reel off five straight games to take the second set 6-3 and force a decider.
Gauff regrouped impressively in the third, taking a 3-0 lead before a long break in play as a spectator received medical treatment.
When play resumed, Gauff wasted no time wrapping up victory, breaking Cirstea twice more to win in 2hr 3min.
"I think overall I'm most proud of just mentally staying in there," Gauff said.
"As the match got longer I was able to raise my level," added Gauff, who will face either sixth seed Amanda Anisimova or 12th seed Belinda Bencic in the quarterfinals.
Czech 13th seed Karolina Muchova strolled into the quarterfinals with a 6-0, 6-2 rout of Alexandra Eala of the Philippines, advancing in just one hour.
Unseeded American Hailey Baptiste is also into the last eight after dispatching 25th seed Jelena Ostapenko 6-3, 6-4, unleashing 11 aces on her way to victory.
Baptiste will next face either world No 1 Aryna Sabalenka or China's Zheng Qinwen, who were playing later.