Andy finished joint leader in the US Open Series together with Nadal. They have 145 points.

Andy, however, cannot win, because the rules for ties are in favour of Nadal (head-to-head in the tournaments involved), so he ends up being runner-up.

What this actually means is that he'll get some extra prize money at the US Open:
Wins - $500,000
Final - $250,000
SF - $125,000
QF - $62,500
R4 - $35,000
R3 - $20,000
R2 - $12,500
R1 - $7,500