As the sun rose on Thursday on another sultry morning in Miami, many of the city's fun-loving inhabitants were still sleeping off the previous night's cocktails. The hedonists exist in a different world - and different time-zone - to Andy Murray. As dawn broke, the Scot was already winding down from the first of his three daily training sessions.
Given that the ATP tour runs from January to November, you might expect to find the world's top players lying on a sunbed somewhere. Yet for Murray, this precious time over Christmas represents the most arduous part of the year.
After squeezing in a brief holiday in Dubai with girlfriend Kim Sears, he has returned to what he knows best: the early-morning runs along the beach, the weight training that has turned a scrawny teenager into a contender for Mr??Universe, and the battles with a fiendish instrument of torture called the VersaClimber.
"A lot of the boxers and UFC fighters use it," Murray explained. "You're climbing with your arms and legs and we have it hooked up to computers so we can see how hard I'm working. It's the hardest thing I do."
While Murray might groan at the prospect of another beasting at the hands of his fitness trainer Jez Green, there is an upside too, because this is the most sociable time of his year. During one technical tennis session this week, there were 10 people on and around the stadium court at Key Biscayne, including his coach Ivan Lendl and his hitting partners Jamie Baker (the world No?246) and Ollie Golding (No?439).
During the morning's work-out, the two journeymen pros lined up on the far side of the net to Murray, looking like a couple of zookeepers trying to corral a 500lb gorilla. Even outnumbered, he was more than a match for them. The afternoon featured "endurance speedwork" - the next-worst thing to the VersaClimber - as Green laid down eight markers on the court and Murray ghosted around them following a pre-determined sequence of movements.
"We've been using the Hawk-Eye stats from my matches at this year's Australian Open," said Murray. "If the first point was 20 seconds then you follow the trail for 20 seconds. You have the 25-second break in between, and then the next point might be 40 seconds. You're moving around the court, doing short sprints, but not actually hitting a ball. Now we've got this data it helps us to make our training more specific."
Miami has been Murray's winter training base for the past five years, ever since he moved into his penthouse flat overlooking the art deco grandeur of South Beach.
While his training schedule concentrates on technique and physical conditioning, he believes there are psychological benefits, too. "It can give you an edge mentally if you know you've started your training early," he said. "I've felt the most comfortable when I've gone into tournaments having had time to prepare."
Doesn't he sometimes fancy enjoying a normal Christmas lunch, rather than sweating his way through 90 minutes of Bikram yoga? (That is the hardcore version where you are expected to extend one toe above your head in a studio heated to 40 degrees.)
Isn't there ever a desire to ease off for a couple of days?
"Not really," he replied. "Once I get to the age where I might not be able to train as hard, I might think that way. But right now I enjoy it. The work that you're doing isn't always fun but in the evenings it's nice that Jamie and Ollie are here as well. And it's good to be away from tournaments and the pressure they bring. Once you get to the Australian Open, that changes."
This year, there is one new and compelling reason for Murray to psych himself up for the pain. During the 1980s, Ivan Lendl was the prime mover for fitness in tennis, attending 7am aerobics classes in his American base of Connecticut. And while Lendl might have looked a little paunchy when he arrived as Murray's coach in January - the effect of switching his attention to golf rather than tennis - much of the excess weight has fallen off over the past year.
He now has an ongoing bet with Green about who can deliver the best VersaClimber score, with the loser expected to read out a humiliating statement in front of the world's media at the Australian Open.
As a player, Lendl's blank stare and hollow eyes made him look like an extra from Dawn of the Dead. As a coach, he has an almost indulgent, paternal style. "Don't worry if your arm is hurting," he called out on Wednesday, as Golding found his elbow jangling under the sustained impact of Murray's missiles. "Jamie will serve, you sit this part out, then come back in for the returns."
"Our relationship is very honest and open," Murray said. "I think that's why it has worked so well and why both of us are planning long-term. When we got together at the end of last year, we said 'Let's see how the first few months of the year go.' Now it's 'What are we going to be doing in four or five years time?'
"I knew it was working after the Australian Open. The night after I lost the semi-final, he just seemed to know what I wanted. Sometimes in the past when I've lost big matches I know I'm not good company but he made sure that everyone was around me and I wasn't just left on my own.
"We spoke the next day and he said: 'These are things we can work on, I know it's tough to say them today because it was a close match.'?"
Despite a mini-slump during the clay-court season - which had the pundits questioning whether Lendl was the right coach after all - Murray's year picked up momentum during his near-miss at Wimbledon and reached its pinnacle one dark and stormy night in Flushing Meadows.
To some observers, his slightly downbeat reaction to victory over Novak Djokovic - he simply squatted down and placed his hands over his face - was a little disappointing. As John McEnroe said last week, "It would be nice if he could show a bit more emotion next time."
Yet Murray's own memories of that moment are sketchy. After his long and tortuous road to this ultimate goal, he was too caught up in the moment to understand quite what was going on.
"Ollie and Jamie were asking me today what happened immediately after the US Open final, in those 30 or 40 seconds," Murray explained.
"And I don't know. They said 'Do you have the racket for the match?' and I said 'No.' I think I threw it into the crowd but no one can really remember.
"I was so focused during that last game that I went to the wrong side to serve, which is weird because I don't think I've ever done that before. The guys were saying to me 'Maybe it didn't all happen, maybe it was all a dream.'
"One of the best parts was going back to Dunblane afterwards. A lot of people came, which was cool, and I got to see my schoolfriends and some of my teachers. It was weird walking down the high street because when I was a kid, everything seemed so huge. And yet now it's so little.
"I went to my grandparents' house and it was the same thing: I used to remember charging around the living-room and now it's two steps the whole length of the room.
"My whole family was there and it's not often I get to see my whole family at once.
"Since the Olympics and the US Open, I've just found it much easier to go out and about, to walk with my head up. Whereas before I was, like, always head down. I probably felt like I was letting people down.
"I'd been reminded every day for the last five or six years how long it's been since someone won a grand slam. So it's nice, finally, not to have to worry about that stuff any more. I can just play and see what else I can achieve.
"There's going to be downs as well as ups, but I hope I'll be in a position to play for grand slams in the future. The US Open was a huge motivation for me, because I realised after that match that all of the stuff that we do here, that it was all worth it in the end."
Even that hour-long session on the dreaded VersaClimber.