Novak Djokovic said he endured a "physical collapse" as he made a losing return to tennis at the Monte-Carlo Masters.
The world number one was playing just his fourth match of the year so far due to his refusal to get vaccinated against Covid-19.
That decision saw him deported from Australia before the Australian Open in January and not allowed entry to the United States for Indian Wells and the Miami Open.
His return did not go to plan either, with him losing 3-6, 7-6,1-6 to world number 45 Alejandro Davidovich Fokina in his opening match in Monte-Carlo.
Afterwards, he revealed much of that was down to the vaccination decision, with his lack of fitness really catching up with him.
"Physically I completely collapsed," Djokovic admitted. "I couldn't move any more. I didn't like the way I felt in the end. I need to talk with my team.
"He was the better player. I was hanging on the ropes the entire match. I just ran out of gas completely.
"If you can't stay in the rally, not feeling your legs on the clay, it's mission impossible."
Despite Djokovic's vaccination stance, the European clay court swing should provide him with plenty of opportunities to play tennis, and he has assured fans he will be back up to speed quickly.
"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," Djokovic said.
"So of course in those types of conditions and circumstances, then you have to really work two times more than you normally would.
"And again, I played a clay-court specialist. He had a match already on centre court a few days ago.
"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.
"I knew a few days ago when I spoke to you that 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."