World number two Alexander Zverev said he "overachieved" at the Shanghai Masters in reaching the semi-finals amid fatigue caused by his busy competition schedule.
The 29-year-old top seed exited in the last four on Saturday as Spaniard Alejandro Davidovich Fokina pulled off a surprise 5-7 6-4 6-3 win, only his second in eight meetings with Zverev.
Germany's Zverev, who is guaranteed to end the season as world number one, has had little opportunity to rest between the US Open, Laver Cup, Beijing Masters and Shanghai.
"I feel like I've actually, in a way, maybe overachieved this week because of how much tennis I played," Zverev said.
"I carried a little bit of injury, I carried fatigue into the tournament, and I'm OK to lose to a player like this today."
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Repeatedly exposed on the forehand side, Zverev surrendered his first break of the match to an exquisite backhand passing shot on the run as Davidovich Fokina took the semi-final to a deciding set.
Zverev was broken to trail 2-0 in the third set and could not recover control of the tie as the 25th seed advanced to his first ATP Masters 1000 final since 2022.
Having failed to replicate the performance which earned him a straight-set win over Alex de Minaur in the quarter-final, Zverev said he has struggled since his fourth-round win over Alexander Bublik went to a third-set tie-break.
"First, I'm not a robot, I'm a human being, I get tired," Zverev said. "After the US Open, I had one day off in total. I think I played a lot of tennis, which is very, very positive because that means I played well.
"I think even here I had tough matches. I personally feel like I was quite flat since the Bublik match, because I was losing energy.
"I played a phenomenal match against Alex de Minaur, but today I was flat. Even in the first set, even though I won the set, I was flatter, I was not reacting so well. I was returning quite badly.
"We're all human beings, even the best players in the world."
His semi-final defeat is a disappointing exception to a brilliant season for Zverev, who claimed the first Grand Slam title of his career at the French Open in June before winning a second at the US Open in September.
Davidovich Fokina will face fellow Spaniard Rafael Jodar in the final on Sunday.