Helio Castroneves shaded his Team Penske team-mate Will Power following the opening session at the Phoenix International Raceway.
Castroneves turned a 19.58s lap to nudge Power (19.59s) as the Penske runners both unofficially lowered the track record.
Recently named IndyCar Series steward Arie Luyendyk holds the track record of 19.60s which has stood for 20 years.
Castroneves, a Phoenix winner in 2002, said the track feels relatively unchanged after undergoing a slight reconfiguration.
“When you come into Turn 1, it’s like, ‘are you going to turn’?,” Castroneves said.
“After the corner, you’re like, ‘Oh, it’s opening up.’ It’s very interesting on that scenario. Doesn’t mean it’s easy.
“We’re still talking about I don’t know how many Gs, but it’s a lot.
“It’s a very heavy steering wheel, which is going to be even more heavy at night because we are going to put fuel in and things like that.”
Ed Carpenter Racing’s Josef Newgarden had earlier taken the top time with a 19.68s effort before dropping to third fastest as the teams tried out the new Chevrolet and Honda aero kits.
Juan Montoya was fourth in 19.78s ahead of Graham Rahal while Penske’s fourth runner Simon Pagenaud sixth.
Marco Andretti and Scott Dixon were seventh and eighth, recording identical times.
The top eight all lowered the 20s mark with Ed Carpenter (20.008) and Chip Ganassi’s Tony Kanaan (20.043) completing the top 10 of the 21 cars to record times.
After being recently named as Chip Ganassi’s fourth driver for 2016, ex-F1 and one-time Nissan LMP1 driver Max Chilton was 14th fastest, just ahead of the returning James Hinchcliffe.
IndyCar returns to Phoenix for the second round of the series on April 2 for the first time since 2005.