Former world number one Andy Murray says he's "lost respect" for Greek star Stefanos Tsitsipas after a lengthy bathroom break led to frustrations in the opening round of the US Open.

The match pushed five hours but Murray's displeasure became obvious after the fourth set, when Tsitsipas disappeared from court for a bathroom break Murray reckoned was unfair.

When Tsitsipas returned several minutes later, Murray dropped serve, and after falling behind 2-0 in the final set, he vented that "It's never once taken me that long to go to the toilet, ever."

Tsitsipas, who was reportedly changing clothes in hot, sticky conditions, then steamed ahead to claim the final set, winning 2-6, 7-6 (7), 3-6, 6-3, 6-4.

Following the match, Murray said he'd "lost respect" for third seed Tsitsipas.

"It’s just disappointing, because I feel it influenced the outcome of the match. I’m not saying I necessarily win that match, for sure, but it had influence on what was happening after those breaks,” he said.

“I lost respect for him.

It wasn't the only struggle Murray faced during the match; at one point he lost balance in sweat-soaked shoes, leaving splotches on the blue court from his soggy clothing. 

Once a US Open champion, Murray is ranked 112 in the world after a series of health challenges.

Upon hearing of Murray's gripe, world number three Tsitstipas said he hadn't broken any rules.

"If there’s something that he has to tell me, we should speak, the two of us, to kind of understand what went wrong. I don’t think I broke any rules," he said.

ABC/AP