Will Power topped a busy opening practice session for this weekend's IndyCar double-header at Reliant Park in Houston.
The Australian was the 'two' in a Helio Castroneves-led Penske one-two during a sedate opening half of the session, but as the times picked up over the final 20 minutes, the leader changed six times before Power made it his own with a 1m00.4427s lap at the end.
Simon Pagenaud, who was among those to enjoy a brief stint in P1, was left to settle for second-fastest with a time 0.2s down on Power's.
Sebastien Bourdais was third, while Takuma Sato and Tony Kanaan rounded out the top five.
Luca Filippi, making his first appearance of the year in the second Rahal Letterman Lanigan entry, finished the morning 12th-fastest, putting himself just ahead of Marco Andretti.
The only major incident came early on, when Scott Dixon slowed with a mechanical problem and was hit from behind by Graham Rahal, who was able to complete the session after pitting for a new nosecone.
That accident coincided with a brief red flag, which was triggered to allow track workers to cut away a troublesome section of rubber sealant between two sections of the concrete surface.
Pos Driver Team/Engine Time Gap 1. Will Power Penske/Chevy 1m00.4427s 2. Simon Pagenaud Schmidt/Honda 1m00.6956s +0.2529s 3. Sebastien Bourdais KV/Chevy 1m00.7700s +0.3273s 4. Takuma Sato Foyt/Honda 1m00.7901s +0.3474s 5. Tony Kanaan Ganassi/Chevy 1m00.8355s +0.3928s 6. Helio Castroneves Penske/Chevy 1m00.8357s +0.3930s 7. James Hinchcliffe Andretti/Honda 1m00.8914s +0.4487s 8. Scott Dixon Ganassi/Chevy 1m00.8939s +0.4512s 9. Justin Wilson Coyne/Honda 1m00.9191s +0.4764s 10. Ryan Briscoe Ganassi/Chevy 1m01.1070s +0.6643s 11. Mikhail Aleshin Schmidt/Honda 1m01.1261s +0.6834s 12. Luca Filippi Rahal/Honda 1m01.3157s +0.8730s 13. Marco Andretti Andretti/Honda 1m01.3371s +0.8944s 14. Charlie Kimball Ganassi/Chevy 1m01.3920s +0.9493s 15. Graham Rahal Rahal/Honda 1m01.3927s +0.9500s 16. Carlos Munoz Andretti/Honda 1m01.4399s +0.9972s 17. Josef Newgarden Fisher/Honda 1m01.5740s +1.1313s 18. Ryan Hunter-Reay Andretti/Honda 1m01.6523s +1.2096s 19. Jack Hawksworth Herta/Honda 1m01.8558s +1.4131s 20. Sebastian Saavedra KV/Chevy 1m01.9171s +1.4744s 21. Juan Pablo Montoya Penske/Chevy 1m02.0613s +1.6186s 22. Mike Conway Carpenter/Chevy 1m02.5256s +2.0829s 23. Carlos Huertas Coyne/Honda 1m02.8000s +2.3573s All drivers use Dallara chassis