Dixon had led the most laps with 27 and was in front of the field when he was penalized for a pit-lane safety violation. The penalty cost Dixon a chance to win the race and trim Castroneves' lead in the championship standings. Castroneves came to Sonoma with a 31-point advantage.
With 15 of 19 races completed, Castroneves has only one finish outside the top-10 this season. He placed 13th in Brazil, his home country. Castroneves has only one win this year, which came at Texas.
"I still feel it's going to be a battle all the way to the end," Castroneves said. "We've been able to hold on to the lead in the points, and even add to it a little bit over the last couple of races, but I know how tough these guys are. No one is giving up and with four races left. There's so many points still out there. We need to keep on it, keep doing what we've been doing and hopefully we can stay out in front with a strong weekend here at Baltimore."
There are 13 drivers who mathematically remain eligible for the championship. Any driver who trails the leader by 163 points or more following Baltimore will be eliminated from title contention.
Ryan Hunter-Reay, the defending IndyCar champion, is currently 62 points behind Castroneves. Marco Andretti is fourth in points (-70), followed by Simon Pagenaud (-99) and Dario Franchitti (-100).
One year ago, Power came to Baltimore with a 36-point lead over Hunter-Reay. Baltimore was the penultimate race. The series wrapped up the 2012 season two weeks later in Fontana, Calif.
Hunter-Reay won at Baltimore and cut Power's lead to just 17. He went on to win the championship in Fontana, beating Power by just three points.
"Last year, this is where, I think, I won the championship, and it's another pivotal weekend for our championship race for 2013," Hunter-Reay said. "We've got a points gap to close, and this is a huge opportunity for us prior to heading to a double race weekend at Houston (Oct. 5-6)."
Power put his season back on track with a win at Sonoma. It was his first victory since April 2012 in Brazil. He is now eighth in points (-123).
"It was just good to finally get a win," Power said. "Things haven't gone our way a lot this year. It's just been a strange season, and it really makes you appreciate how hard it is to win in the IndyCar Series. But I think we've continued to show that we have a fast car and can run up front, so it was good to break through and get that first win of the year. Now, it would be good to take advantage of that momentum and make it two in a row."
Power won the inaugural race at Baltimore in 2011. He started on the pole and put on a dominating performance by leading 70 of 75 laps. Power finished sixth in last year's race there.
Twenty-four teams are on the entry list for the Grand Prix of Baltimore.
Series: IZOD IndyCar. Date: Sunday, Sept. 1. Race: Grand Prix of Baltimore. Site: Streets of Baltimore. Track: 2.04-mile, 12-turn temporary street circuit. Start Time: 2:30 p.m. ET. Laps: 75. Miles: 153. 2012 Winner: Ryan Hunter-Reay. Television: NBC Sports Network. Radio: IMS Radio Network/SIRIUS XM Satellite.