Rafael Nadal said he was slow and rusty as he suffered a disastrous start to his bid to win the ATP Finals for the first time after crashing to a 7-6 (7/3), 6-1 defeat to Taylor Fritz on Sunday.
"It's about time," the 36-year-old said. "I have less time than him to do what I want to do on the ball. I felt that everything was going so fast."
The year-ending Finals is the only major trophy missing from tennis icon Nadal's list of honours but his straight-sets loss means he will struggle to escape the round-robin stage.
He probably needs to win both his remaining matches against Green Group opponents Casper Ruud and Felix Auger-Aliassime if he is to make the semi-finals.
Nadal, who had not played on the tour since the US Open in September, said he is struggling to rediscover his best form after recent injuries.
"Normally I try to come back in lower events," Nadal said. "Normally you don't need to play at your top to win a couple of matches. Then if you made that happen, when you face the top players, you are more ready."
Nadal said he struggled with the aggression of Fritz and the pace of the surface.
"On most of the points of the match, I was in a defensive position and he was in an offensive position. That's what happened. That's something that was difficult to avoid for me because you need to be quicker on your legs, quicker in your mind."
"That's what's happening when you are coming back," he said.
"On this kind of surface, you need to play very well. You don't have time to think."
Fritz, who is at the Finals for the first time in his career after replacing injured world number one Carlos Alcaraz, gave Nadal a torrid time with some aggressive tennis.
"I did a good job, protecting my serve, holding my serve and I did very well in the tie-breaker," said Fritz on court.
"Once I got through that I just wanted to do the same thing I was doing in the second set, protecting my serve and capitalise on break points when I got them."
Nadal said he was outplayed in the second set.
"At the end, Fritz played very well. I was not able to handle his power," said Nadal.
"Serving like Fritz served, then you are under pressure all the time," Nadal said.
"When somebody is serving that way, on the return he is going for every shot. Just well played from him."
Ruud was also a straight-sets winner over Auger-Alissime, 7-6 (7/4), 6-4, in the day's other singles contest. The Norwegian sits behind Fritz in the standings with the top two from each group progressing to the semis.
The world number four came through an uninspiring encounter in one hour and 51 minutes after claiming his one and only chance to break Canadian Auger-Aliassime's serve in game seven of the second set.
This season's Roland Garros and US Open finalist then served out the rest of the set to claim the win.