With the entire field working on setups for primary tyres, the times were remarkably close in the first 25 minutes.
Felix Rosenqvist, who has taken two poles on the Indy road course, was top for Arrow McLaren with a 1m10.4167s, until with 17 minutes to go he was deposed by Alex Palou of Chip Ganassi Racing, by 0.0343s, who was using all the curbs and some of the grass.
Again, the Rahal Letterman Lanigan cars looked strong, with both Christian Lundgaard and Jack Harvey in the top eight.
Also running strong were the other Ganassi cars, with Marcus Armstrong, Scott Dixon and Marcus Ericsson in the top 10.
Pato O’Ward then jumped to the top with 1m10.2604s in the #5 McLaren, and Armstrong improved to go second, but then Lundgaard grabbed P1 by just 0.0024s on a set of fresh primaries.
With five minutes to go, Palou became the first driver to duck under 70 seconds with a 1m09.9383s, an average speed of 125.545mph.
Patricio O’Ward, Arrow McLaren Chevrolet
Photo by: IndyCar Series
That was three-tenths faster than Lundgaard, and no-one could match that, despite late improvements from Harvey and Team Penske trio Scott McLaughlin, Josef Newgarden and Will Power.
Helio Castroneves’ Meyer Shank Racing lost an engine, pulling off after eight laps with a smoking brand-new Honda.
Qualifying will begin at 9pm BST.