In the morning session, the 36-time winner set a 1min17.51sec lap, a mere 0.02sec ahead of his nearest pursuer, Andretti Autosport’s Ryan Hunter-Reay. However, as times improved in the afternoon, Bourdais pulled a couple of tenths clear, to end the day on a 1min17.06, eclipsing Hunter-Reay’s teammate, Alexander Rossi.

“With the cooler temperatures, there’s a lot of downforce in the car and a lot of grip,” said Hunter-Reay, who has posted top-five finishes in his last three Sonoma starts. “It’s a bit different than when we’re here at the end of the season, when the track temps are hotter and we’re sliding around a bit more.

“It’s great to get back in the car here. It’s a very demanding track and there’s a small window to get it all right. We’re trying to tweak the settings a bit to make that window a bit bigger.”

A third Andretti car, that of Marco Andretti, was third fastest in both sessions, although his engine let go towards the close of the day’s activities, having chalked up 90 laps, while the team’s newest driver, Takuma Sato, was fourth in the morning, fifth in the afternoon.

Four-time champion, Chip Ganassi Racing’s, Scott Dixon turned only 16 laps in the morning, but during his 52 laps in the afternoon session, he threw in a 1min17.39sec lap to end the day fourth fastest. One of his teammates, Max Chilton was sixth fastest, quick enough to narrowly beat Schmidt Peterson Motorsports duo James Hinchcliffe and Mikhail Aleshin.

Bourdais’ rookie teammate, Ed Jones, was confined to just 11 laps in the morning session due to a technical problem, but cut 33 laps, as well as his best lap time, to beat two Ganassi cars of Charlie Kimball and Tony Kanaan in the afternoon.

Morning times

Sebastien Bourdais, Dale Coyne Racing 1min17.51sec Ryan Hunter-Reay, Andretti Autosport 1min17.53sec Marco Andretti, Andretti Autosport 1min17.55sec Takuma Sato, Andretti Autosport 1min17.67sec Alexander Rossi, Andretti-Herta Autosport 1min17.86sec Graham Rahal, Rahal Letterman Lanigan Racing 1min17.92sec Tony Kanaan, Chip Ganassi Racing 1min17.94sec Max Chilton, Chip Ganassi Racing 1min18.03sec Charlie Kimball, Chip Ganassi Racing 1min18.10sec Scott Dixon, Chip Ganassi Racing 1min18.19sec James Hinchcliffe, Schmidt Peterson Motorsports 1min18.25sec Mikhail Aleshin, Schmidt Peterson Motorsports 1min18.37sec Ed Jones, Dale Coyne Racing 1min19.56sec

Afternoon times