The Andretti driver lapped the 17-turn 2.9-mile course in 1m39.3721s, beating his 1m39.9303s effort from the morning session.
The opening two-and-a-half-hour session featured two red flags – both for harmless spin-and-stalls by Felix Rosenqvist of Arrow McLaren and Helio Castroneves of Meyer Shank Racing.
Herta became the first driver to crack the 100-second barrier with a 1m39.9303s effort, an average of 110.489mph.
That was just enough to pip his new team-mate, series sophomore Kyle Kirkwood, by 0.0933s, and the Meyer Shank car of Simon Pagenaud by 0.1875s.
Herta raised the bar in the afternoon session where, unlike the morning’s session, several other drivers broke beneath the 100-second barrier.
His nearest pursuer was Rahal Letterman Lanigan's sophomore Christian Lundgaard, a mere 0.0046s in arrears, despite losing five minutes of track time as punishment for bringing out a red flag with a spin.
Chip Ganassi Racing’s Alex Palou, the 2021 IndyCar champion, was third fastest ahead of another Andretti driver, Romain Grosjean.
Reigning and two-time IndyCar champion, Team Penske’s Will Power, was fastest of the Chevrolet runners, 0.1969s off top spot to narrowly hold an edge over his longtime rival Scott Dixon in the second Ganassi entry.
The Juncos Hollinger Racing team showed superbly, with Callum Ilott clocking in at seventh fastest, and multiple TC2000 champion Agustin Canapino, a rookie in open-wheel racing of any kind, finishing the day 20th.
He did cause one of five red flags this afternoon with a spin and stall, but this could not detract from his on-track effort. Canapino’s best time was little more than one second slower than Herta’s benchmark and around 0.7s slower than Ilott.
Penske's Scott McLaughlin was eighth fastest while Simon Pagenaud, fresh off his Rolex 24 Hours at Daytona win, was ninth fastest for Meyer Shank Racing.
Fastest rookie was former Formula 2 winner Marcus Armstrong, who will campaign the #11 Ganassi entry on road and street courses this year, and cede his seat to Takuma Sato for the ovals. However, he did cause the fifth and final red flag when his car stopped at pit entry at the close of the session.
Aside from Armstrong, Canapino, and Lundgaard, the other reds were caused by rookie Sting Ray Robb (Dale Coyne Racing) and Felix Rosenqvist (Arrow McLaren), the latter of whom also spun this morning.
Kyle Kirkwood’s failure to set a representative time in his Andretti car was due to a failed clutch.
Testing resumes at 9am local (Pacific) time on Friday.
Source: Autosport