Dixon finished behind Australian Will Power to complete a successful weekend of racing after winning yesterday's race.
Castroneves held a 49-point lead heading into this weekend's racing but retired from today's race with a gearbox problem which saw him finish 23rd. Similar issues forced Castroneves back in the field in 18th yesterday.
Dixon takes a 25-point lead into the Oct. 19 finale at Fontana, California.
"Another win would have been nice. We came up a little short," Dixon said after the race.
"Will was being pretty aggressive there he hit us before the restart, he hit me going into four, which spun me sideways and he got the run on me down the back straight.
''It is what it is. They've been super aggressive and trying to take as many points away for his teammate as he could.
"All in all a pretty decent weekend. A good turnaround in points for us but it's going to be a tough week in Fontana. 25 points can go anybody's way."
Dixon's teammate Dario Franchitti was in a frightening crash on the final lap when his car hit the fencing. According to reports he was talking with crew members after the crash but his injuries were unknown.
