Andy Murray has doubled down on his criticism of Stefanos Tsitsipas by joking that his US Open conqueror's bathroom breaks take twice as long as Jeff Bezos' trips to space.

World number three Tsitsipas beat Murray 2-6 7-6 (9-7) 3-6 6-3 6-4 in a thrilling five-set battle in the opening round at Flushing Meadows on Monday.

The opening-day showdown was overshadowed by Tsitsipas' controversial bathroom break ahead of the decisive fifth set – the Greek star spending around eight minutes off court, much to the frustration of Murray.

Tsitsipas also required a medical timeout for an apparent foot problem after losing the third set to 2012 champion Murray, who was far from impressed following almost five hours of action.

Speaking after the match, Murray – who failed to progress beyond the first round of the US Open for the first time in 15 appearances – said he had lost respect for Tsitsipas and suggested his opponent had deliberately attempted to disrupt his flow. 

Tsitsipas defended his lengthy break, insisting he had played by the rules and that he would speak to Murray face-to-face should the Briton wish to take the issue further.

Rather than resolving the matter, however, Murray aimed another dig at Tsitsipas with a sarcastic message on his personal Twitter account on Tuesday, comparing the stoppage to the 10 minutes and 10 seconds it took billionaire Bezos to fly to space last month.

"Fact of the day. It takes Stefanos Tsitipas (sic) twice as long to go the bathroom as it takes Jeff Bazos (sic) to fly into space. Interesting," he posted.

With the win over Murray, Tsitsipas became the 10th active player to defeat all four members of the 'Big Four' – Murray, Novak Djokovic, Rafael Nadal and Roger Federer.

Tsitsipas will meet world number 44 Adrian Mannarino in the second round on Wednesday.