Russian star Daniil Medvedev overcame painful cramps and Alexei Popyrin to battle his way into the Miami Open round of 16.

Medvedev looked in all sorts of trouble against the Australian, but the world number two and top seed survived at the ATP 1000 tournament on Sunday.

Canadian sensation Felix Auger-Aliassime was sent packing by former champion John Isner in Miami.

MEDVEDEV ADVANCES… SOMEHOW

Australian Open and US Open runner-up Medvedev could barely walk during his gruelling three-set win against Popyrin.

Medvedev let three match points slip away in the second set, having led 5-2, before cramp set in.

In humid conditions, Medvedev was forced to rely on his serve as he limped around the court.

"Winning a Grand Slam final in straight sets doesn't feel the way I felt after the match point today," he said. "A match to remember for sure."

"I felt like my legs were not following me anymore," Medvedev said. "The only thing I was thinking about is not to fall down, because if you fall down, I don't think I would be able to get up. There were a few moments I just wanted to lay down and say, 'OK, it's over.' That's the thing I couldn't accept myself to do."

Awaiting Medvedev is Frances Tiafoe after he outlasted 16th seed Dusan Lajovic 1-6 7-5 6-3.

AUGER-ALIASSIME TOPPLED

A new year, but same result for 11th seed Auger-Aliassime, who was beaten 7-6 (7-5) 7-6 (7-5) by Isner.

Isner defeated Auger-Aliassime en route to the championship match in Miami two years ago and the big-serving American repeated the feat.

The only former Miami champion in this year's field, Isner remains unbeaten in tie-breaks at Hard Rock Stadium, winning all 11 since the tournament relocated in 2019.

"It was a big test for me. There's probably not many places in the world where I like my chances against Felix," Isner said. "I'm kind of lucky to play him here in Miami, where I like it a lot. If I were to play him on clay somewhere in Europe, it'd probably be a different story."

Next up is seventh seed Roberto Bautista-Agut, who beat Jan-Lennard Struff 4-6 6-3 6-2.

Elsewhere, Jannik Sinner, Taylor Fritz, Emil Ruusuvuori and Alexander Bublik also advanced.