“I struggled, starting to feel very dizzy,” added the 24-year-old Sinner.
“I was very, very flat, you know, the whole body. I don’t remember last time I felt this weak.
“I woke up this morning, didn’t feel very well and tried to keep the points very short. In the beginning I was hitting very clean, very good, and then I just kind of hit the wall.”
Sinner revealed he began to fade halfway through the third set as temperatures once again climbed above 30C during a record-breaking heatwave in France.
But he refused to blame the conditions for his unexpected exit, his earliest at a Grand Slam since he lost at the same stage here in 2023.
“It was warm, but not crazy warm,” said Sinner. “Really it was nothing against the heat, nothing against the weather. It was just me today, but it happens.
“I don’t want to take anything away from him (Cerundolo). He played a very solid match.”
Sinner arrived in Paris on the back of winning all three of the clay-court Masters titles, with his odds of completing a career Grand Slam enhanced further by the absence of injured rival Carlos Alcaraz.
Instead, it was another bitterly disappointing ending at Roland Garros for last year’s runner-up who held three championship points 12 months ago before losing to Alcaraz.
“It’s tough to accept because of the position I’ve been in and everything considered, but yeah, now I have a lot of time to recover,” said Sinner.
“I won’t play any tournament on grass before (Wimbledon). Now I need really some time off, recover completely, also mentally, and then be ready to go again.
“As I said, in the beginning of the year, this (the French Open) is my main goal here. A very early exit it was not what I was looking for.”
For Cerundolo, who goes on to play Martin Landaluce in the last 32, it was just the third Grand Slam match win of his career.
The Buenos Aires native didn’t blink as Sinner tried to gamely hang on, Cerundolo putting away the struggling top seed to pull off a monumental upset.
“It’s tough for him (Sinner). I think I was a little bit lucky,” said Cerundolo. “The match was almost lost.”
“I started to notice that something wasn’t good, because I mean, he was beating me pretty easy,” he added.
“Tennis is about being in the present day and being better in that match. He’s number one in the world because he’s the best every day. (It) just happened this day. I was better this day.”
Remaining men’s top 10 seeds at French Open
2 – Alexander Zverev (0 grand slams, three finals)
3 – Novak Djokovic (24 grand slams, 38 finals)
4 – Felix Auger-Aliassime (0 grand slams, 0 finals)
8 – Alex De Minaur (0 grand slams, 0 finals)
10 – Flavio Cobolli (0 grand slams, 0 finals)