Jannik Sinner, the world No 1, was leading by two sets and serving for a place in the third round of the French Open when it became clear he was in significant danger on the baking clay in Paris on Thursday afternoon. He was undoneat the only grand slam he has yet to win, not by the ingenuity of his opponent or even the immense pressure that comes with being the prohibitive favourite, but rather by his own body.

The Italian – who had not lost a match since February – said he had been struggling with an illness. Despite waving away suggestions he had wilted in the high temperatures, the unprecedented heatwave in the first week here could not have helped his cause.

He fell 3-6, 2-6, 7-5, 6-1, 6-1 to Juan Manuel Cerúndolo before a stunned, packed audience on Court Philippe-Chatrier, the crowd veering between cowering from the sun and being astonished at the sight of a historic result unfolding. Sinner could barely walk and was completely uncompetitive in the second half of the match.

His defeat represents one of the most shocking results in recent years. The 24-year-old had entered the court on a 30-match winning streak and this month became the only man other than Rafael Nadal, the greatest clay court player, to win all three clay court ATP Masters 1000 titles in the buildup to the French Open. In the absence of his great rival, Carlos Alcaraz, last year’s champion who is sidelined indefinitely due to a right wrist injury and will also miss Wimbledon, no one has shown they can consistently match Sinner’s level.

However, Sinner is renowned for his frailty in the heat and the decision to schedule his match at midday led some to wonder whether he would be able to handle the conditions. When he began to fall apart physically, most people naturally assumed he was struggling with the heat. He insisted appearances were deceptive and that only illness was the source of his misery. “This is the sport. It was warm, but not crazy warm. I feel like it was quite OK to play. It was nothing against the heat, nothing against the weather. It was just me.”

Players are used to disruptive weather here coming in the form of rain delays and damp courts, but this year the heat has changed the state of play. The temperature has ranged between 32 and 35C most afternoons.

Such conditions are so rare in the second grand slam of the year that there was significant confusion about the tournament’s heat rules at the start of the week. When asked about them on Wednesday, Novak Djokovic incorrectly thought they did not exist. “

The tournament organisers use wet bulb sensors to monitor the temperature and a match could be suspended when it crosses 32.2C. No match has been suspended here due to the heat.

The scorching temperatures also make a significant difference to the way matches are played on clay. Wet, damp and colder conditions make balls heavier, leading to longer rallies and a lower bounce, the hot weather hardens the court and allow the ball to fly more quickly. The faster courts this year suit more offensive players, aiding serving. Those capable of generating heavy topspin, such as the retired 14-time champion Nadal, are even more dangerous with the ball bouncing high.

As with many things, the players are split on just how difficult this heat has been to handle. The 26th seed, Jakub Mensik, described the conditions as “insane to play in” after collapsing with cramps and having to be escorted towards the locker room in a wheelchair after his five-set second-round win over Mariano Navone on Wednesday.

Despite being well known for his durability, the two-time French Open finalist Casper Ruud completely overheated in the first round and looked on his way out before recovering in the fifth set to defeat Roman Safiullin. “I felt at times really dizzy and just really tired and walking around like a zombie almost,” Ruud said.

As Sinner said, not everyone agrees these conditions are particularly extreme. Competing in the heat is a standard part of tennis, a sport that essentially follows the sun around the globe. Ben Shelton, the fifth seed, noted that the US players have an advantage this week: “In general, we’re more used to dealing with it. All of us live in Florida, so that physical part becomes less of a factor.”

After closing out her 7-6 (1), 6-4 second round win over Donna Vekic, the four-time grand slam champion Naomi Osaka, who also spent her formative years in Florida, said she only wished it could be hotter: “I kind of like it. I want them to turn it up a little bit.”