After the US Open Federer has clinched his spot for the ATP Finals and Del Potro hasn't (see the news here). However, Del Potro has more points than Federer in the Race to London. How can this be? Live ATP Race says Federer has qualified despite not having reached the cut; why is that?
I presume it is due to
- If there is [exactly] one current-year Grand Slam champion positioned between eighth and 20th in the Race, he qualifies in eighth place.
The only other 2018 Grand Slam winners are Nadal and Djokovic, and neither of them can fall below 7th now. So Federer will qualify with a finish of 20th or better (apparently guaranteed at this point).
Quote from a recent article on the ATP website: Djokovic, Federer Clinch 2018 Nitto ATP Finals Spots (Wayback Machine):
Why Does Federer Qualify?
Once Djokovic set a US Open final with Del Potro, Federer clinched his berth because of the 'Grand Slam Champion rule', which reserves one place in the eight-man field for a current-year Grand Slam champion who finishes between Nos. 8-20 in the ATP Race To London. Federer, the reigning Australian Open champion, may finish in the Top 7 and not need to lean on that rule to qualify. But he is guaranteed to finish the year in the Top 20 of the Race.
If Djokovic wins the US Open final, then Federer is the only Grand Slam champion not already guaranteed of finishing in the Top 7. If Del Potro wins the US Open final, the Argentine will have enough points in the Race to qualify on points alone, again leaving Federer as the only Grand Slam champion not already with enough points to guarantee a Top 7 finish. But the Swiss would still be guaranteed of finishing within the Top 20.