The 20-year-old needed eight match points to win 6-3, 2-6, 6-4, 1-6, 7-6(11) after four hours and 41 minutes on Court Six, completely exposed to the sun.
"It's insane to play in this weather, especially in front of the sun, to be there for more than four and a half hours, it's just insane," Mensik told reporters.
"Even with the breaks, you don't have that much time, the ball boy can't bring you a towel.
"During the changeover, you have just one minute, which obviously by the time you sit, it's already just 30 seconds, so there's not much time to cool yourself down."
Mensik collapsed onto the clay straight after hitting the winning forehand.
When his opponent came to greet him, he tried to get up but remained stuck on his back. The umpire waited several seconds before coming to his aid, and eventually, medical staff arrived to help him.
"The referee today, what happened after the match, I will keep to myself, but the behaviour of that... I don't respect that," he added without elaborating further.
Mensik accepted that "rules are rules", but compared the strict regulations forcing players to play quickly compared to the speed with which umpires will halt play when a spectator is taken ill.
"I was really struggling on the last point," he said.
"When I hit the last point, the last winner, the emotions went out, and of course, my body just turned off, so at the time, I wasn't able to do anything."
Mensik will be back on court on Friday to face eighth seed Alex de Minaur, who has not played since Monday after receiving a second-round walkover when opponent Alexander Blockx pulled out with injury.
"Even if it looks horrible at the end, after 30 minutes in the locker room when I was out of the sun, I started to feel much better," said the 26th seed.
"I'm here, feeling pretty well. So, of course, I'll use the recovery day as well as I can. The conditions aren't going to be as crazy as today."