It is part of the charm of any sport that a contest apparently heading in only one direction can suddenly be transformed, often with no obvious explanation. While the Centre Court crowd were experiencing something similar yesterday, it happened twice in the same match on Court One before Novak Djokovic, seeded second, saw off the 18-year-old qualifier Bernard Tomic 6-2, 3-6, 6-3, 7-5 to reach his fifth successive Grand Slam semi-final.
If there was neither rhyme nor reason behind one dramatic change in the second set and another midway through the third, perhaps rhythm was an answer. Djokovic lost his, then recovered it to win seven successive games at a crucial stage of the match. Or was it all a matter of experience, the older man holding his nerve as well as his serve when it mattered, while those of the youngest quarter-finalist for 25 years failed him?
"We had kind of waves, I played better and then he played better," was as close as Djokovic could get to an answer. "After the first set I relaxed a bit and he got confident. I played one terrible service game but I wasn't worried because I knew I could get back."
Tomic is one of that new breed of cosmopolitan sportsfolk, in his case of Croatian stock, born in Germany but brought up in Australia, where he will now succeed Lleyton Hewitt as the country's No 1. "I'm proud of myself, I gave it as much as I could," he said. "I had my chances at 3-1 in the third and could have been a bit smarter. A result like this tells you I'm only a few matches away from winning titles. Hopefully I can win a major in the next two years."
If spectators were expecting a mere warm-up to Rafael Nadal's match, the very first game must have had them wondering whether the champion would be on court before dark. It lasted almost 10 minutes, full of long baseline rallies, the final one of which was cut off after 22 strokes by one of the drop shots Djokovic would try with mixed success throughout. It meant a service break and the pattern seemed to be established, confirmed with a second break by the Serb for 5-2 and an ace to take the set in just over half an hour.
Cue the first change of direction after another long rally won by Tomic, as Djokovic's serve suddenly went to pieces. He could hardly have been more generous with the fourth game of the second set had it been handed over the net in wrapping paper and ribbon; getting the ball there was the problem, two double faults plus two unforced errors offering an opportunity that the teenager capitalised on by holding to love for a 4-1 lead. He closed it out at 6-3, then immediately broke to win the first game of set three.
Having arrived at this stage of the tournament with fewer games lost than any other survivor, and only one defeat in 2011, Djokovic was in crisis against an opponent ranked 156 places below him, who had begun the afternoon as a 10-1 outsider. Yet from 3-1 down, came the second transformation of the match. Tomic, rushing a service game when he needed to take deep breaths and pause, lost it to love, saw the set disappear 6-3 and was 2-0 down in the fourth before he regained some composure.
A third switch-back? Breaking back and leading 5-4 it was conceivable, but proved illusory. Another gorgeous drop shot allowed Djokovic to break with a scream of triumph and serve out for the match before an embrace of genuine warmth between the two regular hitting partners.
Djokovic retired to the locker room to observe the stunning victory of Jo-Wilfried Tsonga, of whom he said: "It was an amazing comeback. He's been playing great in the grass-court season so far. I expect a very even match [in the semi-final]."