As far as Novak Djokovic is concerned, this is the big one. As far as men's tennis is concerned, it doesn't actually get any bigger than this.
The Serbian has one chance to make history - history that we have not seen for 52 years and will probably not see again. He knows it, and we know it too.
It's very simple from Djokovic's point of view: If he beats Daniil Medvedev in the US Open final, he wins the calendar Grand Slam and establishes himself as the most successful male tennis player in history - and almost certainly the greatest.
Djokovic v Medvedev Head-to-head
Djokovic wins: 5
Medvedev wins: 3
Although Djokovic has the best of the head-to-head with Medvedev, the Russian has actually won the majority of their last five matches.
They have come on a variety of surfaces too. Medvedev beat him at the Monte Carlo Masters on clay, the Cincinnati Masters on an outdoor hardcourt, and the ATP Finals on an indoor hardcourt.
What he has never done, though, is beat Djokovic in a best-of-five match. He has twice had the opportunity - both at the Australian Open - and he has only won one set of the seven they have played.
Their last Grand Slam meeting was a big one too. It was the Australian Open final at the start of this year. Medvedev went into it confident, but Djokovic comprehensively outclassed him.
That is likely to have left its mental scars for Medvedev, but there is also some context. First, Djokovic is supreme at the Australian Open. His record there is one of complete dominance. Secondly, Medvedev finishes seasons much stronger than he starts them.
Novak Djokovic in focus
World ranking: 1
Turned pro: 2003
Career W/L: 978/197
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
SF: Alexander Zverev 4-6, 6-2, 6-4, 4-6, 6-2
Djokovic was pushed all the way in the semi-finals against Alexander Zverev, who forced the world number one into a fifth set.
There is an inevitable question of how much physically that has taken out of Djokovic. He has only got the job done in straight sets once in the tournament so far, which is very unlike him.
At 34-years-old, that may catch up with him, but the context of the match and what's at stake means he will almost certainly be able to push through it.
There is the mental question too, about whether or not the sheer pressure of the situation will take its toll. It has on Serena Williams before. She has lost four major finals since hauling herself to the verge of a historic 24th major.
Djokovic's other problem is he is up against a seriously high-class opponent. So he has plenty with which to contend, but he is Novak Djokovic…
Daniil Medvedev in focus
World ranking: 2
Turned pro: 2014
Career W/L: 205/96
US Open so far
R1: Richard Gasquet 6-4, 6-3, 6-1
R2: Dominik Koepfer 6-4, 6-1, 6-2
R3: Pablo Andujar 6-0, 6-4, 6-3
R4: Dan Evans 6-3, 6-4, 6-3
QF: Botic van de Zandschlup 6-3, 6-0, 4-6, 7-5
SF: Felix Auger-Aliassime 6-4, 7-5, 6-2
Medvedev is yet to win a major but he is very much the real deal. This will be his third major final and he has been unfortunate to have run into Djokovic and Rafael Nadal in the other two.
However, he is world number two, he is the reigning ATP Finals champion, having beaten both Djokovic and Nadal on the way. He also led Russia to the ATP Cup in Australia earlier this year too.
Medvedev is also very much a hardcourt specialist. All of his significant successes have come on that surface. In truth, that means he is unlikely to ever take the mantle from Djokovic as a complete player who dominates on all surfaces, but on this particular one he is a huge threat.
It has to be said that he has had a favourable draw in the US Open this year. Ever since Stefanos Tsitsipas was eliminated in round three, there hasn't been a genuine obstacle in his path to the final.
Still, it means he will step onto court against Djokovic feeling confident and fresh, and it may be the edge that he needs.
Novak Djokovic to win 3-0
Let's get this one out of the way right now. Djokovic was at the very top of his level when these two met in the Australian Open final, and when he is there he is untouchable.
He beat Medvedev in straight sets and it was a veritable beat-down, and it absolutely could happen again.
Djokovic was even carrying an injury that day.
Daniil Medvedev to win first set 7-6
Medvedev will surely have learned plenty from the experience at the Australian Open, and for all Djokovic was at his best that day, the Russian was also significantly short of his best.
Even then, the first set was a tight, and it is likely to be again in this one.
Djokovic has dropped too many first sets for his liking recently, so back Medvedev to steal this one on a tiebreaker.
Daniil Medvedev to win 3-2
Medvedev can absolutely win this match, but if you're going to beat Djokovic in a Grand Slam the likelihood is you'll need all five sets to do it.
If it does go the distance, this is where Medvedev's extra freshness could come into it. The Russian enjoys long rallies too, so even getting it to a fifth might take a lot of out of Djokovic.
Normally, the longer the match is the more you'd back Djokovic, but this one feels different.