I do agree with Mr Shackelford that tennis does go off the national stage after the US Open. However, to the players, there's still plenty to play for after the Open--there are two (2) 1000 series tournaments in Shanghai and Paris before the season ending event in Torino. If you're a player, you simply can't afford to miss those 1000 series events for three specific reasons: (a) the money's certainly pretty good, (b) as a player, you can't afford to earn zero points in a 1000 series event by not showing up and (c) by performing well, it absolutely will assist a player avoiding certain difficult, early round match-ups at the Australian Open in January. By Djokovic missing Indian Wells and Miami, those results alone will preclude him from reaching no. 1 at year's end unless Alcaraz picks up a knock before the US Open.
The season ending tennis championship seems missing because it has moved from New York to London to Shanghai to Torino.
From what I understand about player contracts, for those around number ten in the world, these tournaments are vital in terms of what one could make off the court if finishing in the top ten at season's end.
In tennis, there was a reason why Jim Courier proposed his redistribution of the Grand Slam purse money to the earlier rounds. If one's at one hundred in the world, you're barely scraping by at years' end. And, one hundred doesn't guarantee one admittance into any 1000 series events, where the maximum admittance is ninety six. With the advent of the 250, 500 and 1000 series, it rewarded those at the top and made it harder for those below than, say, seventy five in the world. Before, the top guys had to participate in the traveling circus; now, not so much. The top four, five can live with the Slams and the 1000 series events alone.