Kei Nishikori felt tiredness and nerves cost him in his three-set loss to Marin Cilic in the US Open final on Monday.
The Croatian 14th seed was too strong for Japan's Nishikori, claiming a convincing 6-3 6-3 6-3 victory at Flushing Meadows.
Nishikori had come into the clash after five-set wins over Milos Raonic and Stanislas Wawrinka, as well as a tough victory over world number one Novak Djokovic.
It finally appeared to catch up with him in the final, and Nishikori said he felt "heavy" during his first grand slam decider.
During his deep run, Nishikori said he learned his body could cope with testing encounters.
"I played three long matches. I played two five sets and another three hours, two hours, against Novak," Nishikori said.
"And now I'm here with my body is heavy still, but I can able to play. I think few years ago it didn't happen like this.
"I think I showed my potential. I can beat anybody now. So if I can keep training hard and also practice hard, I think I have more chance coming up."
Nishikori had no expectations heading into the final grand slam of the year after a toe injury in the lead-up.
He said his performance in the final was his worst of the tournament.
"There is a lot of positive things that I learned from these two weeks," Nishikori said.
"It was one of worst matches I played today, but also he was very aggressive and very fast.
"I was, I have to say, a little bit nervous. A little bit. First final. I mean, even semis. But very excited these two weeks.
"I didn't expect nothing coming here. Before I was injured and little practice. There is so many positive things for sure that I can have from these two weeks beating Stan and Novak again. Disappointed of course today, but very good two weeks."