Jannik Sinner is stunningly out of the 2026 French Open in the second round after one of the all-time collapses in sports.

The top seed and No. 1-ranked player in the world was up 6-3, 6-2, 5-1 Thursday against Juan Manuel Cerundolo — one game away from victory — before losing 18 straight points and ultimately dropping the final three sets 7-5, 6-1, 6-1 while dealing with cramping issues in the 90-degree weather in Paris.

Sinner, who exited the court during the third set, lost 18 of the last 20 games as he completely unraveled, though he did not blame the heat for his issues.

Sinner said he woke up Thursday not feeling well, and that continued on the clay.

“I struggled, starting to feel very dizzy, very low on energy,” he said.

The four-time Grand Slam winner used bags of ice and a fan to try to cool down during the match.

“It was warm but not crazy warm,” Sinner said. “I feel like it was quite OK to play. Really it was nothing against the heat, nothing against the weather. It was just me today, but it happens.”

Sinner, 24, was the heavy favorite to win his first French Open as Carlos Alcaraz is out with a wrist injury.

The French Open is the only grand slam Sinner hasn’t won; the Italian lost to Alcaraz in a fifth-set tiebreak during last year’s epic final.

Cerundolo, 24, is ranked No. 56 in the world and is unseeded in this tournament. He has reached the third round of a grand slam for the first time in his career.

“I think I was a little bit lucky,” Cerundolo said in an on-court interview after his upset of Sinner. “I feel sorry for him.”

This marks the earliest Sinner has been ousted at a grand slam tournament since he lost in the second round at Rolland Garros in 2023.

The loss snapped a 30-match win streak for Sinner, who is the first men’s No. 1 to lose before the third round at the French Open since Andre Agassi was upset by Slovak Karol Kucera in 2000.

Sinner is the first top-seeded male in the Open Era (since 1968) to suffer multiple losses at a grand slam after holding a 2-0 set lead, according to ESPN.

He will have to regroup ahead of the start of Wimbledon in a month.