The 2026 French Open is just in its fifth day, and there has been no shortage of drama. However, the most shocking upset happened early on Thursday morning. Manuel Cerundolo upset top-ranked Jannik Sinner in a five-set comeback.The heat — a factor all week — left Sinner physically weakened by the third set, and his game fell apart. Sinner tried to deflect attention from his physical fitness during his post-match press conference to avoid detracting from Cerundolo's victory. However, the media in Paris hounded Sinner for answers and were able to extract insight into what exactly went wrong for the four-time Grand Slam champion. Below is a breakdown of Sinner's post-match press conference.What Sinner Felt in Third SetWhen asked what he felt when he was up 5-1 in the third set, Sinner explained that he woke up this morning not feeling well and seemed to hit a wall at the worst possible time in the match. "Struggled. Started to feel very dizzy. Very low energy and tried to serve it out, but didn't have a lot of energy. Fourth set, I let it go a little bit, trying to have a bit more energy in the fifth. Very important game, the first one, couldn't hold, and then it went all a bit downwards, but yeah. Woke up this morning, didn't feel very well, and tried to keep the points very short. Also in the beginning, I was hitting very clean, very good, and then I just kind of hit the wall, and that's it."Sinner Doesn't Blame the HeatSinner explained that he began feeling weak in the middle of the third set, but refused to blame the heat. "It was warm, but not crazy warm," Sinner said. "I feel like it was quite okay to play. It was nothing against the heat, nothing against the weather. Was just me today, but it happens."Sinner Didn't Sleep WellSinner said there were multiple factors behind his stamina giving out. The biggest issue was his lack of sleep. "There's not only one. I played a lot. Didn't have a lot of time to recover." He later added, "I felt this morning I didn't sleep very well. This morning when I woke up, I was struggling a bit, but this can happen. Usually in Grand Slams you always have couple of days where you don't feel perfect. This was today. And yeah, this happened."Grass Court SeasonDespite defending his title at Wimbledon, Sinner does not plan to play any grass-court tournaments to prepare for the Championships. " I won't play any tournaments on grass before, most likely. Now, I need really some time off to recover completely, also mentally, and then be ready to go again from Wimbledon."The French Open runs through June 7. Stay locked into Sports Illustrated's Serve On SI for all of your tennis news from the court and beyond.Add us as a preferred source on GoogleFollow