The question of whether anyone can stop Novak Djokovic from winning the calendar Grand Slam at the US Open has dominated the discussion all tournament, and he's just about to run into what always looked like the most likely man to do it.
The world number one will play Alexander Zverev, the big-serving German who ended Djokovic's Olympics dreams just a month ago.
That doesn't make Zverev the favourite to win this semi-final by any means, but he certainly has a genuine chance of doing something no one else has done this year: beat Novak Djokovic over five sets.
Djokovic v Zverev head-to-head
Djokovic wins: 6
Zverev wins: 3
Novak Djokovic has a positive head-to-head against every active player that he has ever played on the ATP Tour. That's not a generalization, it's a genuine fact.
Zverev, though, has made a better fist of facing him than most, and most consider the German to be the biggest threat remaining to the calendar Grand Slam.
That calendar Slam may have even been a golden slam (all four majors plus an Olympics gold medal in the same calendar year) had it not been for Zverev, who beat Djokovic in the semi-finals in Tokyo recently.
Zverev's other two wins against Djokovic have come in finals, and he has beaten the Serbian on all three surfaces on which they have played.
Basically, Rafael Nadal and Roger Federer aside, Zverev provides as good as a head-to-head against Djokovic as you are going to see.
Novak Djokovic in focus
World ranking: 1
Turned pro: 2003
Career W/L: 977/197
US Open so far:
R1: Holger Rune 6-1, 6-7, 6-2, 6-1
R2: Tallon Griekspoor 6-2, 6-3, 6-2
R3: Kei Nishikori 6-7, 6-3, 6-3, 6-2
R4: Jenson Brooksby 1-6, 6-3, 6-2, 6-2
QF: Matteo Berrettini 5-7, 6-2, 6-2, 6-3
We tipped Djokovic to lose the first set against Berrettini but win 3-1, and that is exactly how that one transpired.
Djokovic has not been starting matches well at the US Open, which may show there is some tension in his arm early on.
Why there should be, though, is anyone's guess. During this tournament Djokovic has underlined his status as the unrivalled master of five-set tennis, and that was evidenced against Berrettini too.
The longer the match goes on, the stronger Djokovic gets, and if you're going to beat him you're likely going to have to do it over at least three-and-a-half hours of tennis.
Alexander Zverev in focus
World ranking: 4
Turned pro: 2013
Career W/L: 294/136
US Open so far:
R1: Sam Querrey 6-4, 7-5, 6-2
R2: Albert Vinolas Ramos 6-1, 6-0, 6-3
R3: Jack Sock 3-6, 6-2, 6-3, 2-1retired
R4: Jannik Sinner 6-4, 6-4 7-6
QF: Lloyd Harris 7-6, 6-3, 6-4
Alexander Zverev has been the 'chosen one' for years on the ATP Tour, with him tipped for the very top pretty much from the start of his career.
He is yet to deliver on that in terms of major titles, although he did reach the final at the US Open last year.
What he has done, though, is won plenty of other titles, with Olympics, ATP Finals, and Masters crowns all sitting nicely on his record. He has beaten Djokovic on the way to winning all three.
There can be little doubt, too, that Zverev is currently playing the very best tennis of his career so far. He is the biggest and the best server in the men's game right now, and his backhand is seriously elite too.
The big question over Zverev has been his second serve. On pressure points, his second serve can crumble and the double faults flow. He has found a solution, of sorts, in often choosing to serve two first serves instead, and it has worked well for him before.
That tactic could seriously help him against Djokovic, as it puts added pressure on his return. Both Zverev and Daniil Medvedev have used it against him before with success. It's not a guarantee, by any means, but it increases his chances.
Alexander Zverev to win the match
You might want to go old school on this one and keep it simple.
Ordinarily, betting against Novak Djokovic to win a match makes little sense. Djokovic is the ultimate predator: you might take a set from him, you might cause him problems along the way, but sooner or later he's going to get you.
With Zverev things don't look that simple. The German is genuine capable of beating Djokovic and doing it on the Serbian's own terms. He's got the quality.
Novak Djokovic to win 3-1
Djokovic has won four of his five matches in this year's tournament so far by three sets to one, so it's a solid bet just on that alone really.
You get the feeling, though, that the players might share the opening two sets, making the third absolutely crucial. Whoever wins it will likely close it out quite quickly.
So, with that in mind…
Alexander Zverev to win 3-1
Zverev might just be the only player on the ATP Tour who does not actually fear Djokovic.
He has beaten him in finals, he has beaten him on different surfaces, he has beaten him from behind, and he has beaten him in straight sets.
That may count for a lot in the crunch moments, and if they go his way then he's well capable of making them count.