Starting second on the grid, Alice Powell swept up the inside of Spa winner Emma Kimilainen to take the lead on lap 6 at turn 10.
Jamie Chadwick had looked sure to catch her fellow Brit in the points, but Powell had built up a substantial lead at the 20-minute mark and crossed the line 2.8 seconds ahead after 20 laps.
The two title contenders both sit on 109 points with two races to go at the Circuit of the Americas and Mexico City. Powell has three wins to Chadwick's two. Nearest rival Kimilainen sits 34 points behind the pair
See a full recap from the weekends W Series event: