The world No. 2 made it to the Monte Carlo Masters quarterfinal on Thursday with a 6-1, 6-7, 6-3 win over Czech player Tomas Machac, but the game revealed a weakness that Jannik Sinner hardly displays. The Italian breezed through the first set in 30 minutes but stumbled in the second. He lost two breaks of serve and proceeded to lose the set in a tiebreak 7-3. What went wrong?

Sinner was seen touching his back repeatedly and had called the physio during the match. He had even taken a medical timeout. Likewise, Machac also needed his elbow to be treated, leading to a chaotic middle stretch. In the third set, the second seed somehow discovered another level. He broke early and closed out the match in two hours. This was the first time Sinner had lost a set in his last 18 Master level matches. After getting the win through struggles, he was candid about it. 

“It was a tough match, I was struggling a bit in the second set, I was a bit tired today. I hope I can recover for tomorrow,” Sinner said in his post-match interview.

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“I felt great before the match. In the second set I struggled a bit to find the right energy, but this can happen. I tried to push myself through, which I have done. Even when you don’t feel your best, you try to find ways and that was the case today and I am happy. The main priority is to recover,” he added.

The admission carries weight. Sinner has been consistent in 2026, winning the Sunshine Double at Indian Wells and Miami, and going 12 matches without a loss. Now, going to Monte Carlo as the player with the most to lose after narrowing the ranking difference between world No. 1 Carlos Alcaraz to only 1,190 points. A title in Monaco would be a big step back to the number one position that he last occupied five months ago.

But the fatigue he mentioned is not surprising. Sinner has been playing a colossal amount of tennis lately, winning Indian Wells, winning Miami, and even going into the doubles draw here in Monaco with Zizou Bergs before dropping out before their second round game. The physical strains of such a schedule are starting to mount.

Despite the health concerns, Sinner was measured about the challenge ahead. “I am happy to be in the quarterfinals, it will be tough against FAA next round,” he said, referring to sixth seed Felix Auger-Aliassime, a player he leads 4-2 in their head-to-head.

The quarterfinal is set for Friday. Between now and then, one question dominates: how well can the Italian recover?

Felix Auger-Aliassime Awaits in the Quarterfinal

Despite all the worry about the physical condition of Jannik Sinner, a step backward would be worthwhile to see what Tomas Machac really ended on Thursday. Sinner had won 37 straight sets in ATP Masters 1000 events before the Czech took the second set tiebreak, a streak of domination that had been months long. But it is a reminder that even a Sinner is human.

The question now is whether he can get back on track and be in time to play Felix Auger-Aliassime, a player who has been gaining his own momentum in Monaco. The sixth seed has been in fine form this week, playing his part in his part of the draw with the same disciplined violence which renders him dangerous on clay. He defeated the 2014 US Open champion, Marin Cilic, in the R32 and received a walkover mid-match in the following round against Casper Ruud. His booming serve, strong forehand, and upgraded clay court record (54% win ratio) make him a real threat to anyone on a particular day.

The head-to-head, however, tells a familiar story. Sinner is leading 4-2 in their career meetings, and the Italian has always managed to counter the most potent guns of the Canadian. 

The top position in the world is still in sight. A victory in the quarter final on Friday keeps Sinner well on course. A defeat, especially with a seeded opponent, is an opening for Alcaraz to solidify his status as the leader. 

Should Sinner sleep well and wake up feeling better than he did on Thursday, Felix will have a very hard afternoon ahead of him. In case the weariness persists, the best of Montreal will stand behind himself to make an upset.