Andy Murray's mental toughness has never been in doubt but his victory over Yoshihito Nishioka in the U.S. Open first round on Tuesday delivered the surest sign yet the Scot is also physically capable of going the distance at a major once more.

The 33-year-old Briton, who has twice had hip surgery, battled back from two sets down to win 4-6 4-6 7-6(5) 7-6(4) 6-4 in a gruelling four hour, 39 minute marathon in his first singles appearance at a Slam since the 2019 Australian Open.

Murray's last appearance at a major ended in tears as he fell to Spaniard Roberto Bautista Agut in an epic five-setter in the first round in Melbourne, with the Scot headed for surgery and an uncertain future.

There were no tears on Tuesday, however.

"I've just played a four-and-a-half hour match when I never thought I'd be able to," Murray told reporters.

"I felt way better today at the end of that match than I did when I played Bautista in Australia. I'm not sitting here with my hip throbbing and aching. I'll be able to sleep fine tonight."

Murray, who recovered from a break down in both the third and fifth sets and saved a match point late in the fourth, said he needed to fine-tune his game now that the questions over his fitness had been answered.

"I was very close to going out but kept coming back, kept fighting, so I'm proud of that," he added.

"The biggest question would have been the physical one. That was the thing I was most happy with, that I lasted.

"That was probably my biggest doubt going in and the biggest answer I got from the match today, was physically that I was good. Tennis-wise I could do better."

Next up for Murray, who claimed the first of his three major titles at Flushing Meadows in 2012, is Canadian 15th seed Felix Auger-Aliassime.