The German top seed was beaten 5-7, 6-4, 6-3 by Davidovich Fokina, who secured just his second victory in eight career meetings with the 29-year-old.
Zverev pointed to fatigue after a demanding run of tournaments, having had little time to recover following the US Open, Laver Cup, Beijing Masters and Shanghai Masters.
“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.”
Zverev said his fatigue became more noticeable after his gruelling fourth-round victory over Alexander Bublik, which went to a deciding-set tie-break, and he struggled to recover from that match.
“First, I’m not a robot, I’m a human being, I get tired,” he added. “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.”
2026 was a breakthrough year for Zverev, as it saw him claim the first Grand Slam titles of his career at the French and US Opens.
