Murray's four-set defeat by Federer yesterday was a testament to the Swiss player's improved fitness, his own back finally free of pain and enabling him to play some of his best tennis once more, at the grand old age of 32.
The good news is that Murray reported no real pain in what was his first significant tournament since he underwent back surgery four months ago and he should be fit to face the United States in the Davis Cup World Group first-round tie in San Diego, starting a week tomorrow.
The switch to clay is far from ideal for his back, as it recovers from the rigours of best-of-five-set tennis, but he has committed to the tie as Britain try to see off a USA side comprising John Isner, Sam Querrey and Bob and Mike Bryan in the doubles. It is a task almost as big as winning the first grand slam event of the year proved to be. That was a step too far, a little too soon, as it turned out.
Had the tournament fallen a couple of weeks later, Murray might have felt better equipped to take on the world's best, but he admitted he just didn't have enough matches under his belt to make him feel comfortable, in body and mind.
"A lot of work went into this slam compared with other ones where you have a few weeks to prepare," said a disappointed Murray. "This time I had a long time to prepare, maybe just not enough matches."
Murray had been telling anyone who would listen that the chances of him winning the title were all but zero, even if he was striking the ball well and looking fit.
Practice and competition are two totally different animals and against Federer, who came out firing in the opening two sets, Murray lacked the sharpness we are so used to, his usually peerless backhand not quite firing.
A lot of the credit has to go to Federer, who came forward at every opportunity, turning defence into attack with ease and putting away anything short. "That's what I used to do so well, the transition game from defence to offence," Federer said, having won 49 of 66 points at the net. "I definitely sensed that today, I am back, physically. I'm explosive out there. I can get to balls. I'm not afraid to go for balls. Last year, at times, I couldn't do it, but the important thing is that I can do it now. I'm looking forward to the next match."
Much has been made of the addition of Stefan Edberg to Federer's team, and some of his sneaks to the net, especially off returns, were reminiscent of the Swede at his stealthy best, even if the real benefit of their relationship will probably only become clear in the coming months.
The larger racket head the Swiss is now using also looks to be making a difference, offering easier power on serve and, crucially, a bigger sweetspot to help with returns.
Murray was caught off balance time and again in the first two sets, looking up to see Federer already at the net, with an easy volley.
Having dominated the first two sets, his only really weak moment came when he served for the match at 5-4 in the third set, as he handed Murray back the break with a number of errors.
Murray sensed his chance but found himself 5-2 down and then faced two match points at 6-4 down in the tiebreak, but lifted his game to save them both and win it 8-6 to extend the match into a fourth set.
In normal circumstances, the momentum would have been with a fully-fit Murray and the Scot might have expected to go on to at least level the match. Yesterday, though he survived a 19-minute second game to hold serve and stayed on serve to 3-3, the signs were there that he might not last the distance, at least not at full throttle.
Federer said he sensed that Murray was running out of steam, perhaps feeling the strain. "Right off the bat in the fourth set I realised, he's not that fast, maybe things are getting a bit difficult for him now," he said. "Not knowing if it was actually the back or just fatigue because maybe he couldn't work out as much as he wanted to."
Murray didn't exactly sound upbeat when asked about playing on clay - with all the demands that places on his back - in San Diego. "It's not perfect for rehabbing a back surgery," was all Murray would say. "Ideally I'd stay on the same surface."
Rather bizarrely, Murray is then entered to play on clay again in Acapulco at the end of February, before switching to hard for Indian Wells and Miami in March and April. It is a schedule that could yet change but, while he decides, Federer takes on Nadal for the 33rd time, but the first time in a grand slam since the same stage here two years ago, when the Spaniard won in four sets.
Nadal has been struggling with a raw blister on his left hand but Federer said he expected a tough match. "The head-to-head record is in his favour," Federer admitted, "but I hope I can make it to the final. Clearly when you're in the semis you start dreaming. ."
Nadal, who missed last year's event through injury, squeezed into the semi-finals at the expense of the Bulgarian Grigor Dimitrov, who had three set points to take a two-sets-to-one lead but eventually went down 3-6, 7-6, 7-6, 6-2.
The impressive showing by Scotland's Isabelle Wallace in the junior event ended yesterday when the Scot was beaten by Jana Flett of Croatia in the last 16 of the singles, fading after the first set to a 3-6, 6-1, 6-1 defeat. Earlier, she and Hungary's Fanny Stollar, seeded fifth, were beaten by the No.4 seeds, Anastasiya Komardina of Russia and the Serb Nina Stojanovic.