Even through the muslin anaesthetic of a television replay thousands of miles away the day afterwards, it was not just that Rafael Nadal took less than an hour and a half to beat Roger Federer on one good leg that induced quiet disbelief. What was the greater surprise and revelation was the rare sight of the Swiss rolling over, albeit with a back twinge that hampered his movement.
The most telling verdict was Nadal's. "The second set was strange," he said of his 6-4, 6-2 win over the defending champion in the quarter-final of the first ATP 1000 event of the season. "Roger didn't fight as usual. Probably he had some problems and he didn't feel enough comfortable to keep fighting."
And you imagine he was trying to be kind.
"I'm happy to be out there and able to compete," Federer said, "but it's obviously a small issue. That doesn't work against guys like Rafa, obviously."
If his back problem, which surfaced earlier in the week but did not prevent him beating Stanislas Wawrinka, was indeed "a small issue", Federer would have cause to worry, because it begs the question: what, then, was wrong?
Twice at 3-3 in the first set of their quarter-final, after being 40-15 up, Federer tried to serve and volley on second serve – inventiveness or desperation? He dropped serve, lost the set and went on to fade in the second.
There were moments when he plainly declined to hunt down 50-50 balls, especially along the baseline. The physical struggle was apparent, as Federer's peerless, single-handed backhand could not cope with the climbing top-spin of Nadal's ground strokes, but Federer looked to be suffering inside as well. He moved dolefully between points, head hung, no hint of resistance in his eyes.
The fans packed the main court to witness a contest between the two players who had dominated the game for the first decade of the millennium, and were served up an anti-climax – and a puzzle. Because, while this was not the Nadal of a year ago but a recuperating version of that glittering talent, it was also a pale imitation of Federer. They littered the match with a mix of mistakes and occasional trademark brilliance in the fight that never was.
Pointedly, Federer and Nadal had not met this early in a tournament since Miami nine years ago; we are used to seeing them collide in the big matches, semi-finals and finals. Yet, in several respects, this otherwise insignificant result might prove to be a turning point in both the careers of the two players and the seemingly irreversible change at the summit of the game.
A year ago here, Federer beat Nadal in the semi-finals. It fitted the order of things, the Swiss prevailing on his favourite surface against the king of clay. Back then, though, the tennis landscape was markedly different, with burgeoning evidence of the upheavals to come.
Everyone was aware a changing of the guard was already under way. Novak Djokovic's brilliant 2011 run of 43 unbeaten matches hit an unstoppable rhythm on this court, and 2012 started well enough, too. The Serb ceded the No1 spot to Federer from July to October, but reasserted himself in Melbourne.
Is Federer losing his competitive edge? It is difficult to accept. His hunger for titles always seemed insatiable.
Although he took under an hour to dismiss Denis Istomin here in the first round and was as commanding as ever until Wawrinka took a set off him in the third round, he must have realised when the pain spread across his back that a match against Nadal was going to be a minor nightmare.
Nadal, still only 26, is playing in his first hard-court tournament since returning to tennis on the giving clay of South America. That ended a seven-month rest from the game after his chronic knee problems forced him to quit the tour upon losing in the third round at Wimbledon to Lukas Rosol, then ranked 100 in the world.
On the face of it, losing to Nadal should not be a seismic event in Federer's early-season campaign. But the Swiss now leaves the tour for two months, husbanding his resources before the clay season begins in Europe, and hoping that the flickering back pain is no more than that.
He will be 32 in August and earlier this week he said: "Can I be No1 again? I hope so, but things are going to be a bit more difficult this year. I made a schedule according to my needs as a player and as a dad. I need to make sure I train a lot, spend more time in Switzerland, have more off-time, because the last few years have been incredibly busy. That's where this year is a bit more of a transition period. But, if I do play amazing and win some of the big, big tournaments, I obviously can return to be world No1, but it's not as big a priority as it was last year. What matters is that I enjoy it, do have success, because that fuels motivation, inspires you to achieve more. That's where I am today."
It is weird to hear Federer sounding more like a contender than a champion.
Nadal, meanwhile, is rejuvenated. If he rises again on the clay of Europe, if he wins the French Open, he will be back with a bang.
No matter the surface, Nadal and Federer, if fit, are unlikely to lose often to the likes of Tomas Berdych, David Ferrer, Juan Martín del Potro or Janko Tipsarevic, with whom they are so familiar. But it would be a brave bookmaker who would still make either of them favourites to beat Djokovic or Andy Murray.
It is players such as Grigor Dimitrov, Milos Raonic and Bernard Tomic who will smell blood. Whether they are capable of the kill remains to be proved. I doubt it in big matches.
However, where only a couple of years ago there was predictability in tennis, now there is a growing sense of drama in nearly every collision.
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image: © Jimmy Barrett