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."
