Sonoma IndyCar: Will Power fastest for Penske in Friday practice

The reigning champion was on top for most of the afternoon, finally settling on a 1m17.4858s effort that was less than 0.1 seconds off his pole time from last year.

"That was an interesting session," Power said.

"It's the first time we got to try that new [compound] tyre on a long run and it definitely degrades a lot, so I think that's going to create really good racing.

"Obviously [qualifying] tomorrow is what really counts, but I think we're in a good window.

"We'll just go home, study, and see if we can make some good changes to go a little bit faster."

Graham Rahal's hopes of toppling Juan Pablo Montoya's championship bid got off to an encouraging start: the Rahal Letterman Lanigan driver was unhappy with his car early in the session, but climbed up to second once the team had made some changes.

"I think it was a good start for us," he said.

"The beginning wasn't very good; we made some changes from the [recent] test and were struggling pretty bad there.

"But we got the car turned around; made some major changes and the car was a lot better."

Ryan Hunter-Reay rounded out the top three, while Montoya finished up in ninth.

Oriol Servia, who was called up to drive Justin Wilson's #25 Andretti Autosport entry, had an inauspicious start to the day when his car stopped at Turn 2 on his out-lap, prompting the only stoppage of the session, but he later returned to complete a total of 33 laps.

Fellow series returnee Mikhail Aleshin finished the afternoon 17th-fastest in the second Schmidt entry.

FRIDAY PRACTICE TIMES: