(CNN) -- Andy Murray struggled in the Australian heat as he survived a scare against world number 199 John Millman at the Brisbane International Thursday.
The U.S.Open champion, ranked third in the world, appeared to struggle with the humidity after racing through the first set 6-1 in just 30 minutes.
Millman, 23, was playing just his fifth match on the main ATP tour and shocked Murray by taking the second set 7-5.
And despite the Briton constantly clutching his stomach and legs, Millman was unable to repeat his heroics in the final set as Murray prevailed 6-3.
"It's incredibly humid here and I've had a lot of travelling too, and there were so many long points of very high intensity," Murray told reporters.
"It's just taking a little while to adjust to playing matches again after such a long period.
"At the beginning of most years the first few matches can be tough on the body, but you get used to it and you feel better and better.
"That was the perfect first match for me in many ways, to go through some tough moments, a lot of long rallies and close games and important points, and to come through."
Elsewhere, Serena Williams set up a semifinal showdown with world number one Victoria Azarenka.
Williams, the reigning Wimbledon, Olympic and U.S.Open champion, saw off 19-year-old Sloane Stevens 6-4 6-3.
Williams has won 11 of the 12 matches she has played against Azarenka and triumphed in all five games against the Belorussian in 2012.
But the 31-year-old played down her chances of winning the upcoming contest.
She told reporters: "I'm going up against the best player in the world," Williams said. "She's had a fabulous year. She won the Australian Open and has been the most consistent all year.
"I've got nothing to lose. I can go out there and see where my game is at."
Azarenka booked her place in the last four following a 6-1 6-0 win over Kazakhstan's Ksenia Pervak.
"It's a tough match, there is no question about it," she said ahead of the Williams clash.
"I'm looking forward to it. There is one more chance for us to meet (before Melbourne) and it's going to be a great test for the Australian Open.
"I'm not looking for easy matches. The tougher it is the more exciting for me it's going to be, so I'm quite happy with that."
Elsewhere, U.S. star John Isner faces a race against time to be fit for the Australian Open.
The world number 14 withdrew from the Hopman Cup in Perth Thursday after suffering an injury to his right knee.
Meanwhile, home favorite Bernard Tomic built on his shock win over world number one Novak Djokovic by defeating Andreas Seppi.
The 20-year-old Australian eased to a 6-3 7-5 victory to bolster his confidence ahead of this month's grand slam.
"It's an amazing preparation before the Australian Open," he told reporters.
"I have won three matches that are important for the summer of tennis in Melbourne.
"To beat Novak last night and then win again tonight was huge for me."
In the women's section, 16-year-old Ashleigh Barty stunned former French Open champion Francesca Schiavone 6-0 6-3.
"I don't even know what happened out there," Barty said. "It was incredible."