SONOMA, Calif. -- There weren't points on the line during the first day of practice for the GoPro Grand Prix of Sonoma, but championship contenders don't see it that way. The undulating road course could be a game-changer.

There are 20 drivers mathematically eligible for the title entering the 85-lap race Aug. 25, and the top two in points -- Helio Castroneves and Scott Dixon -- were fast out of the gate on the 2.385-mile, 12-turn course. Dixon, who has won three of the past four races in the No. 9 Target Chip Ganassi Racing car and is 31 points behind the front-running Castroneves, topped the time chart with a best lap of 1 minute, 18.3145 seconds in the 45-minute afternoon session.

Click it: GoPro Grand Prix of Sonoma combined practice results || Qualifying groups

Castroneves was .3549 of a second behind in the No. 3 Hitachi Team Penske car. Reigning IZOD IndyCar Series champion Ryan Hunter-Reay, who is third in the standings, was third overall with a lap of 1:18.6814 in the 45-minute morning session.

"Year after year the championship comes down to the last race and we'll just carry on and try to get the most points out of each race," said Dixon, a two-time series champion who won at Sonoma in 2007.

Added Castroneves: "It's wide open; anything can happen."

On Aug. 24, the third practice session begins at 2 p.m. (ET), followed by the three rounds of qualifying for the Verizon P1 Award at 5:30 p.m. (ET). A 30-minute warmup is scheduled for 1 p.m. (ET) Aug. 25, with the 85-lap race at 4 p.m.

Will Power, who set the track record of 1 minute, 17.2709 seconds in qualifying in 2012, was fourth overall in practice (1:18.7454) in the No. 12 Verizon Team Penske car. The top 14 on the combined practice time sheet were separated by one second.