Novak Djokovic walked off Centre Court at Wimbledon on Tuesday night at six minutes to eleven — the curfew deadline — having just completed the longest quarterfinal in the tournament's recorded history. He had been on court for five hours. He had saved a match that slipped away in the fourth set, forced a deciding tiebreak, and won it 7-6(4) against a Félix Auger-Aliassime who pushed him as hard as anyone has pushed him on grass in years. The final scoreline: 7-6(10), 3-6, 6-3, 6-7(4), 7-6(4). It was extraordinary. It was also worrying, because the man waiting in the semifinal is Jannik Sinner.

Djokovic is 39 years old. He has won this tournament seven times. He is chasing a 25th Grand Slam title that would be the most any player in the history of the sport has won. Those facts sit together in a kind of tension that the sport has not had to process before — the record is within reach, but so is the physical reality that five-hour quarterfinals on grass courts do not come free. When asked afterward whether he wished he could have played shorter, he joked that it would be nice to play ninety minutes like Lionel Messi. The line went everywhere. The laugh it got from the crowd was real and earned.

He was also fined for the match. The specific violation cited was related to conduct during the encounter — Djokovic has a history of on-court outbursts and code-of-conduct incidents at Grand Slams, and Wimbledon's officiating has not given him a pass in recent years. The fine is unlikely to register as anything other than a footnote given the scale of what he produced, but it is part of the complete picture.

Auger-Aliassime, for what it is worth, was not merely a compliant opponent riding along in someone else's legend. The Canadian pushed this match into territory that unsettled Djokovic repeatedly, took the second set comfortably and forced a fourth-set tiebreak that he won convincingly. There will be a long conversation in his camp about how close he was, and how different this looks if that fifth set goes the other way.

The semifinal against Sinner is the story now. Jannik Sinner is the world number one, and has been performing with the kind of controlled, pressure-absorbing tennis that makes him extremely difficult to beat in a best-of-five format on any surface. On grass, his game has matured significantly — the serve, always strong, has become a weapon; the return has sharpened; and his ability to stay composed in tight moments, which was once his greatest attribute, has become almost mechanical in its reliability.

The head-to-head between Djokovic and Sinner tilts toward the Italian in their recent meetings, and former British number one players have been forthright in saying they do not believe Djokovic can win in his current physical state after Tuesday's exertion. That is not an unreasonable read. Djokovic will have a day's rest at most before they walk back onto Centre Court, and five hours of grass-court tennis at maximum intensity is not something a body of any age simply absorbs and moves on from.

And yet — this is the caveat that must exist whenever Djokovic is written off — his record of defying exactly this kind of conventional wisdom across a twenty-year career is essentially unmatched in the sport. He has won Wimbledon before after taxing fifth sets. He has beaten world number ones in Grand Slam semifinals on minimal recovery. The age is new. The stakes are not. Whether the body and the will can still align the way they once could is the only real question, and Centre Court on Thursday will answer it.