The Scot suffered heatstroke in the 40C-plus temperatures and needed extensive treatment after he and Australian partner John Peers won their first-round doubles match at Melbourne Park.
It was not clear whether the elder Murray brother would be able to carry on in the tournament even with a day of rest.
After beating Vincent Millot in his second-round singles match yesterday, Andy Murray said of his sibling: “I think he’s doing better. He went back to the hotel earlier. I spent an hour or so with him when I got here, and then checked up with him a couple of times.
“But he wasn’t in a great way. He was struggling for a good three or four hours after the match.
“It’s never happened to him before. When the cramps happen — I don’t know how dangerous they are, but they’re very, very uncomfortable.”
Andy has played many better matches than his second-round win, but has never had a better finish.
The world number four was taken by surprise when French qualifier Millot suddenly began to play well above his ranking of 267, smashing winners from all sides as he raced into a 5-1 lead in the third set. Millot had a set point but Murray saved it, and from then on won every single point — 23 in succession — to win 6-2, 6-2, 7-5.
He said: “It was 6-5 when I went to serve for it, and someone shouted out, ‘You won 19 in a row’. I would say that’s probably the most I’d ever won in my career by far.
“It’s very difficult to do. I don’t really know how I did it. But I didn’t realise until I went to serve for the match. It was a good way to finish.”
One man who won’t have to worry about the heat any longer is fifth seed Juan Martin del Potro, who lost to Spain’s Roberto Bautista Agut in the upset of the tournament so far.
The Argentine had been tipped as a dark horse for the title and was expected to meet Rafael Nadal in the quarter-finals.
But Bautista Agut, the world No 62, came from two sets to one down to triumph 4-6 6-3 5-7 6-4 7-5 as the clock ticked past 1am.
Del Potro said: “I think he played a great match for the four hours. It’s tough when someone plays at a very high level for four hours, tough to beat the opponent.
“I was close. But at every moment that I had a chance, he played an unbelievable shot.”
In the women’s draw, Maria Sharapova had to battle for more three hours in roasting conditions to reach the third round, then called for clarity on the tournament’s extreme heat policy.
The third seed was on court for three hours and 28 minutes in her 6-3, 4-6, 10-8 win over Karin Knapp.
Sharapova said: “The question I have is no one really knows what the limit is. We have never received any emails or warnings about the weather or what to do. Actually, I did receive one while I was in the ice bath a few minutes ago, and I was like, ‘that’s a little too late’.