John McEnroe believes extreme heat remains Jannik Sinner's "kryptonite" after the world No. 1 suffered one of the most astonishing collapses in French Open history.
Sinner's hopes of lifting a maiden Roland Garros title were shattered on Thursday when the Italian squandered a commanding lead against Juan Manuel Cerundolo before crashing out in the second round.
The four-time Grand Slam champion appeared to be cruising into the third round after taking the opening two sets and moving 5-1 ahead in the third.
Instead, the match unravelled in dramatic fashion.
Sinner began struggling physically in the sweltering Paris conditions and eventually slumped to a shock five-set defeat, losing 18 of the final 20 games as world No. 56 Cerundolo completed a remarkable comeback.
The stunning result immediately sparked debate about what had caused the collapse, with tennis great McEnroe pointing to one factor above all others.
Speaking on TNT Sports US, the seven-time Grand Slam champion suggested the soaring temperatures had once again exposed one of the few weaknesses in Sinner's game.
John McEnroe believes extreme heat remains Jannik Sinner's "kryptonite" after the world No. 1 suffered one of the most astonishing collapses in French Open history
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Jannik Sinner had been in cruise control before his remarkable collapse at the French Open
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"I was watching [Moise] Kouame after the first couple sets, the young 17-year-old French kid who made a great effort to win in five, but I didn't anticipate at 6-2, 6-3, 5-1, which is absolute cruise control that this thing would turn out to be historic," McEnroe said.
"Not by any means!
"Until you started seeing this, because this is his kryptonite Patrick [McEnroe], there's no question about it.
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French Open winners since 2000 | GETTY/GB NEWS"The heat is a lot of people's, I hated playing in heat like this, but it is a big-time equaliser and it turned out to be the difference maker in this match."
Temperatures exceeded 30 degrees during the contest, with Sinner visibly struggling as the match slipped away.
The Italian later rejected suggestions that the heat alone was responsible, insisting illness was the primary reason behind the collapse.
Sinner had arrived in Paris as the overwhelming favourite after winning 30 consecutive matches and establishing himself as the dominant force in men's tennis.
The 24-year-old looked certain to extend that run before Cerundolo staged one of the most unlikely comebacks of the season.
While much of the attention has focused on Sinner's exit, Cerundolo was quick to acknowledge his good fortune after recording the biggest win of his career.
Jannik Sinner struggled in searing heat as his hopes of French Open glory came crashing down | GETTYThe Argentine, who reached the third round of a Grand Slam for the first time, wrote on X: "Wow, what a match haha, no doubt I got pretty lucky but super happy with the victory.
"Thanks to everyone for the support."
Sinner's defeat has dramatically transformed the men's draw, removing the tournament favourite before the end of the opening week.
The door is now open for the likes of Novak Djokovic and Alexander Zverev to capitalise.