Castroneves on pole at Fontana IndyCar race
Brazil's Helio Castroneves during practice for the IndyCar World Championship Race on August 29, 2014 in Fontana, California - by Robert Laberge
With Friday's pole victory, Castroneves moved from 51 points to 50 behind Power in the battle for the 2014 IndyCar series title.
"It ain't over. It's just beginning," said Castroneves, who is aiming for his first championship. "Our job is to be aggressive over the 500 miles and finish where we started."
Castroneves posted a two-lap average speed of 351.6 km/h (218.5 mph) during Friday's qualifying.
Juan Pablo Montoya qualified second with an average speed of 349.7 km/h. Josef Newgarden placed his Sarah Fisher Hartman Racing car on the outside of row one. This race, on a 3.2 kilometre oval track, will feature a three-wide start.
Castroneves, 39, claimed the 41st pole of his career, to move past Rick Mears for fourth place on the all-time list.
It also marked his third pole of the season and the third time he has won the pole at this southern California facility. He also started from the pole in Fontana in 2002 and 2003.
Power needs to finish sixth or better Saturday to clinch the series championship, regardless of what Castroneves or anyone else does.
"With the starting position, I'd rather start at the very front or the very back to stay out of trouble early on," said defending Fontana race champ Power. "It's obviously all about the race. It's very long, and we'll be racing in different conditions."
Power's 21st pole finish is a season-low for him. Last year, he earned the pole with a record average speed of 355.2 km/h.
New Zealand's Scott Dixon won an IndyCar race in Ohio earlier this month after starting in the last spot in a 22-car field.
Simon Pagenaud, the only other driver who remains eligible for the title, qualified in 13th place. Pagenaud trails Power by 81 points.
Takuma Sato, 2013 IndyCar champion Dixon, and Charlie Kimball will start from the second row.
Tony Kanaan, rookie Mikhail Aleshin and Ryan Hunter-Reay will roll off from row three.