Andy Murray described his mindset for the rest of the US Open as "playing with the house's money" after his narrow escape against Robin Haase.
The Scot was hit by cramp throughout his body after winning the first two sets of his opening-round match on Monday and admitted he was very close to defeat.
Haase served for the fourth set but could not take advantage of his opponent's troubles.
Murray told the BBC: "I was lucky to come through, I could easily be on the plane home. So I'm playing with the house's money and I'll just try and go out there and do the best I can."
The eighth seed practised in the heat of the day on Tuesday with British No 2 James Ward, who is still hoping to get into the main draw as a lucky loser.
Murray reported no ill effects from his ordeal but will no doubt welcome the fact his second-round match against German Matthias Bachinger will not be played until Thursday, giving him two days to fully recover.
He still does not know exactly why the cramp came on but suspects he got his eating and drinking wrong.
Murray said: "I felt physically ready, it was just a strange situation yesterday. Hopefully I'll pull up fine over the next couple of days.
"Before I went to bed (on Monday) I felt fine, I woke up with no aches or pains or stiffness. That suggests it was more down to something I hadn't eaten or drunk or I was lacking in something before I went out there, rather than me physically not being in good condition.
"I just need to do all the right things. I don't play my next match until Thursday so I'll have time to hydrate and get enough fluid in the body, and eat as best I can."
Murray is the only British singles player left in the tournament after women's number one Heather Watson was blown away by Romania's Sorana Cirstea.
Watson has had a great season and went into the match as the favourite but nothing worked and Cirstea raced to a 6-1 6-1 win in just 54 minutes.
Watson, 22, still has not won a match at Flushing Meadows since winning the junior title in 2009 and will look to put this one behind her when she heads to Asia for the series of tournaments out there next month.
She said: "It's disappointing leaving after a first-round loss here, especially after my year and the confidence I've gained, but it's just another week and I'm going to have a lot more weeks."
The headlines on Tuesday were made by 15-year-old American Cici Bellis, who stunned Dominika Cibulkova in her first grand slam match.
Bellis is the youngest player to win a US Open match since Anna Kournikova in 1996 and came from 3-1 down in the final set to triumph 6-1 4-6 6-4.
There were straightforward wins for Wimbledon champion Petra Kvitova, Eugenie Bouchard and Ana Ivanovic while two-time runner-up Victoria Azarenka came from a set down to defeat Misaki Doi.
Roger Federer turned on the style in a straight-sets win over Australia's Marinko Matosevic while the standout result of the day session in the men's draw was 17-year-old Borna Coric knocking out 29th seed Lukas Rosol.