Sergio Garcia finished with a 68 to finish second on a windy day at Valderrama.
Westwood was one of only six players to finish under par after a 70 put him three strokes ahead of Order of Merit leader Robert Karlsson and longshot Miguel Angel Jimenez.
Karlsson initially struggled with his putter but rallied for a 73.
The greens were so quick that if you just missed a few then you could also lose your confidence, Karlsson said.
Three-time major winner Padraig Harrington, Karlssons closest competitor for the European Tours money title, was three shots behind him.
Westwood and Harrington could win the Order of Merit with a victory coupled with Karlsson finishing no higher than fourth. Jimenez also needs to win and have his three competitors finish near the bottom to secure the title.
I dont think you can feel comfortable around this golf course, but I have the patience to go out there and work for my shots, Westwood said. Its a grind, but I enjoy grinding. I do feel like it suits me more.
Harrington reached only two of the first nine greens in regulations and tough chips at No. 2, 4 and 5 resulted in bogeys as he made the turn at 3 over without a birdie.
Its been proven before that its not hard to come back on this golf course, Harrington said. We have 54 holes of golf to play, the tide will turn at some point.