World number two Novak Djokovic admitted the physical strain of remaining unbeaten in 36 matches this year is starting to take its toll.
The 23-year-old struggled with a knee problem during his marathon three-hour 6-1, 3-6, 7-6 (7/2) semi-final victory over Andy Murray at the Rome Masters on Saturday.
That left him less than 17 hours to recover ahead of Sunday's final against world number one Rafael Nadal.
"There's no secret that physically it's catching up with me definitely," said Djokovic, who is six matches away from equalling John McEnroe's record season start of 42 wins set in 1983.
"I've been playing the most matches of any players this year, next to Nadal, but playing at a high intensity and wanting to win every match.
"It's normal and natural that you get tired or exhausted after matches like this, it's nothing new to me."
And he joked: "What I've done wrong is playing 40 matches in four months."
Djokovic came within two points of losing when Murray served for the match at 5-4 in the decider.
"Whoever saw the match knows that the winner was not decided until the last point," he said.
"It was a very close match, a fantastic match to be a part of. He was playing really well, serving really well, especially in the second set.
"He put a lot of pressure on my service games, he wasn't making a lot of unforced errors, I had to earn my points.
"Physically it took a lot out of both of us, it was a great effort but unfortunately one of us had to lose.
"I didn't wait for mistakes to come, I was going for my shots, I made a couple of good shots, a couple of important returns, he made two double faults in that game at 5-4.
"There was a lot of pressure involved, it's not easy to handle."