Casper Ruud said he felt he was almost "walking around like a zombie" at times as he struggled in extreme heat during his first-round win over Roman Safiullin at the French Open.
Temperatures were said to have reached 33C on the first two days of the tournament in Paris, causing uncomfortable conditions for some.
Norwegian Ruud spurned five match points in the third set against Safiullin, by which time he said he had already started to feel cramping in his legs.
He later had medical timeouts and used ice towels and water to try to cool himself during changes of ends.
From 5-2 down in the third set, Russian qualifier Safiullin won 11 consecutive games to force a decider - but he also had injury problems which required treatment towards the end of the fourth set.
Both players left the court for an extended period before the fifth set, after which 15th seed Ruud eventually completed a 6-2 7-6 (7-5) 5-7 0-6 6-2 victory. The contest lasted three hours and 56 minutes on Court Simonne-Mathieu.
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Ruud, a two-time runner-up at Roland Garros, said: "As we know, there's a bit of a heatwave at the moment and that can sometimes cause problems.
"It felt like it was a bit of a kind of heatstroke feeling. I experienced something similar some years ago when I played in Washington DC and I had to retire in the third set because I had that - that's the only time I had that same feeling as I had today in the fourth set where I felt at times really dizzy, really tired and walking around like a zombie almost.
"Luckily, I was 2-1 up still and allowed myself to kind of lower the intensity a bit to get my pulse and body temperature down as much as possible in the fourth to see if there was any chance to finish in the fifth and have some extra energy. Luckily, that ended up working."
Asked if he felt it was a mental victory or physical victory, the 27-year-old said: "It feels like a mental win.
"At times in the fourth [set] I was thinking 'I have to book the flight home tomorrow and I'll be watching from home on the sofa the next two weeks'. Luckily, that's not the case.
"Physically, also, I'm proud because I never really gave in. I didn't give up."
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