NEW YORK -- After his first-round loss to Stefanos Tsitsipas on Monday at the US Open, Andy Murray said his opponent had unfairly gained an advantage by taking a lengthy bathroom break and an opportune medical timeout.

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

Tsitsipas won the tightly-contested marathon match, 2-6, 7-6 (7), 3-6, 6-3, 6-4, in four hours and 48 minutes. The No. 3 seed and 2021 French Open finalist received medical treatment on his left foot after losing the third set and went to the bathroom after winning the fourth set.

Murray complained on the court about how long Tsitsipas was taking in the toilet at the time and again once play had resumed. He remained frustrated when speaking to the media following the match.

"The issue is that you cannot stop the way that that affects you physically," Murray said. "When you're playing a brutal match like that, you know, stopping for seven, eight minutes, you do cool down. You can prepare for it mentally as much as you like, but it's the fact that it does affect you physically when you take a break that long, well, multiple times during the match.

"I think when he took the medical timeout, it was just after I had won the third set. Also in the fourth set when I had Love-30, he chose to go -- I don't know if he changed his racket or what he was doing. But, yeah, it can't be coincidence that it's happening at those moments.

"I don't believe [his foot] was causing him any issue at all. The match went on for another two and a bit hours after that or something. He was fine, moving great I thought."

Tsitsipas said he didn't break any rules and that he was changing his clothes in the bathroom.

"I don't think I broke any rules," he said. "I played by the guidelines, how everything is. Yeah, definitely something for both of us to kind of chat about and make sure. I don't know how my opponent feels when I'm out there playing the match. It's not really my priority.

"As far as I'm playing by the rules and sticking to what the ATP says is fair, then the rest is fine."

Murray is the second player in less than two weeks to accuse Tsitsipas of unfairly gaining an advantage through bathroom breaks. After Tsitsipas took an eight-minute break following the first set in his semifinal against Alexander Zverev at the Cincinnati Open, Zverev told the chair umpire he believed Tsitsipas was cheating and receiving improper coaching via his cell phone.

"He took his bag with his phone and everything in it," Zverev said on the court. "This was the same thing in Paris and is going to be the same thing every other tournament he's playing."

When asked about Zverev's accusations, Murray would not speculate on if Tsitsipas was receiving coaching during his break and said he had "no idea" if that was happening. Murray did, however, say Monday's actions changed his opinion on Tsitsipas.

"I think he's a brilliant player," Murray said. "I think he's great for the game. But I have zero time for that stuff at all, and I lost respect for him."