Herta topped the 45-minute session around the 11-turn, 1.786-mile Exhibition Place street course with a best lap of 1m00.5657s.
With rain threatening, the track was busy right from the start of the session as teams tried to get runs in before the showers took hold.
Herta set the early pace at 1m03.5437s before Scott Dixon (Chip Ganassi Racing) and then Romain Grosjean (Andretti) usurped him from the top of the speed chart. Grosjean set 1m02.1538s, which was soundly beaten by team-mate Kyle Kirkwood’s 1m00.8111s.
Local hero Devlin DeFrancesco set an impressive P2 for Andretti, 0.6279s off Kirkwood, with David Malukas (Dale Coyne Racing) leaping up to third.
Despite intensifying rain drops, Arrow McLaren’s Felix Rosenqvist rose to second, 0.2799s slower than Kirkwood, while Graham Rahal jumped to fourth for Rahal Letterman Lanigan Racing.
After 15 minutes, Dixon claimed second place with 1m00.8652s, just 0.05s away from Kirkwood’s best time, with Grosjean and Herta swapping third spot.
Source: Autosport