World number one Novak Djokovic explained that he could just not get up to the level of sharpness required in his shock defeat at the Monte Carlo Masters.
Djokovic slumped out in the second round on Tuesday when he lost 6-3 6-7 (5-7) 6-1 to world number 46 Alejandro Davidovich Fokina.
The Serbian's status as an unvaccinated player has limited his ability to compete on the ATP Tour this year and he has played in just one tournament so far, getting to the quarter-finals in Dubai.
Yet his preparations for the French Open, which starts at the end of next month, hardly got off to a good start as he made 51 unforced errors in what was still, despite his rustiness, a surprise defeat.
"I didn't like the way I felt physically in the third. I just ran out of the gas completely," Djokovic told the media.
"Just couldn't really stay in the rally with him. I mean, if you can't stay in the rally, not feeling your legs on the clay, it's mission impossible.
"I'm going to look with my team into reasons why that was the case and go back to the drawing board. Hopefully next week will be better in Belgrade."
Though Djokovic has won twice in Monte Carlo, it has not been a happy hunting ground for him in recent years. He has not advanced past the quarter-final stage since 2015.
He added: "It's going to take some time for me to really feel my best on the clay.
"That's historically always been the case. [I have] never played very well in the opening tournaments of the clay season.
"But it's okay. Obviously it is what it is. I have to accept the defeat and keep working."
While unhappy with his own performance, Djokovic had nothing but praise for his opponent.
"I would like to congratulate Alejandro. He was the better player," Djokovic said. "He managed to find a better rhythm I think in the first two sets. I was hanging on the ropes the entire match. I was really chasing the result constantly.
"I always believed that I could come back and win the match, and I stayed there even though a lot of things were against me in terms of how I felt on the court. Game-wise, physically I was just far from my best.
"I expected this match to be [a] really tough match, [a] physical battle, and that's what it was. Unfortunately I'm on the shorter end of the stick, and my week ends here."
Djokovic's defeat also means a mouthwatering contest with teenager sensation Carlos Alcaraz, who he might have met in the quarter-finals, will not be taking place.