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I know we've had years where the underrated but not-highly-thought of Tommy Robredo has made the ATP Year End Finals, but this year we can have a whole bunch of "never thought they'd make it" players.
The current standings (pre-US Open) are (going down to 1500 points):
1. Nadal, 7365 - lock if healthy
2. Federer, 7145 - lock if healthy
3. Zverev, 4175 - pretty sure he's a lock
4. Thiem, 3535 - probably a lock
5. Wawrinka, 3150 - out due to injury
6. Dimitrov, 3060
7. Cilic, 2905
8. Djokovic, 2585 - out due to injury
9. Murray, 2290
10. Querry, 2075
11. Berdych, 1960
12. Goffin, 1900
13. Nishikori, 1885 - out due to injury
14. Pablo Carreno Busta, 1875
15. Milos Raonic, 1795
16. Tsonga 1725
17. Kyrgios, 1690
18. Isner, 1680
19. Sock, 1655
20. Roberto Bautista Agut, 1620
21. Albert Ramos-Vinolas, 1610
22. Muller, 1605
Barring a semis or higher US Open for Murray and a good post-US Open season, I am doubtful about Murray. He should make it, but he hasn't been playing as well as he should for this entire year.
I think it's entirely possible that Querry and/or Goffin make it, and while they may be fine players, I have a hard time thinking of them as making it to the ATP Finals.
If, like me, you are sick of the ATP constantly going on about #ATPNextGen, then you can take some pleasure in the fact that the inaugural Milan tournament is probably going to be missing its top seed. If Zverev makes the real ATP Finals in London, I can't see him going to Milan the next week. Oh, he would probably get a bunch of money and ranking points (though I can't find anything on the site about points, so maybe not), but London offers a lot more. Though, Zverev could consider it an exhibition, which it seems to be with changing the rules.