Nico Rosberg ended the opening day of the US GP fastest of all as Red Bull's Daniel Ricciardo split the championship leader and Lewis Hamilton on the Practice Two timesheet.
Hamilton comfortably topped the morning session at the Circuit of the Americas but lost the initiative to his Mercedes team-mate during the afternoon's qualifying-style runs.
Rosberg's quickest time of 1:37.428 on the supersoft tyres was 0.291s faster than Hamilton's, with Ricciardo wiping out Mercedes' large advantage from the opening session to finish just under 0.2s back on the world champions.
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The later long runs were similarly competitive between Mercedes and Red Bull, offering hope that at least two teams will be in contention for victory this weekend.
"I think Red Bull will be satisfied with their long-run performance," said Sky F1 analyst Paul Di Resta. "They can tune in some more speed overnight and can certainly take the fight to Mercedes."
Ricciardo, who led for parts of last year's rain-hit COTA race, was buoyed by the pace of the RB12: "I think we can find a bit more but I'm sure Mercedes can too. But I'm pretty happy with that. If we had the pace we had today against Mercedes on Sunday it will be a fun race.
While Red Bull moved forwards after a quiet P1, Ferrari slipped backwards with Sebastian Vettel fourth quickest but 0.8s off the ultimate pace.
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Vettel told Sky F1: "They were very quick but it's only Friday. We had a tough day - the Ferrari you saw today is not the Ferrari you will see tomorrow."
Vettel and team-mate Kimi Raikkonen were among a host of drivers to complain that their tyres were going off towards the end of their long runs.
Force India, aiming to increase their points advantage over Williams this weekend, finished in strong sixth and seventh places, just ahead of the McLarens on an encouraging Friday for the Woking team.
After an abject showing at engine partners Honda's home race at Suzuka, the undulating COTA layout appears to be more favourable for the MP4-31 and Jenson Button and Fernando Alonso were separated by less than a tenth in eighth and ninth respectively.
Kimi Raikkonen had been the first driver to sample the fastest supersoft tyres but wound up only 10th after several errors on his quick lap.
With tyre wear proving a challenge throughout the field, a trio of drivers spun off the circuit - Esteban Ocon, Jolyon Palmer and, in the final seconds of P2, Carlos Sainz.
United States GP Practice Two timesheet
1. Nico Rosberg, Mercedes, 1:37.428
2. Daniel Ricciardo, Red Bull, +0.194
3. Lewis Hamilton, Mercedes, +0.291
4. Sebastian Vettel, Ferrari, +0.820
5. Max Verstappen, Red Bull, +0.900
6. Nico Hulkenberg, Force India, +1.150
7. Sergio Perez, Force India, +1.210
8. Jenson Button, McLaren, +1.355
9. Fernando Alonso, McLaren, +1.443
10. Kimi Raikkonen, Ferrari, +1.507
11. Carlos Sainz, Toro Rosso, +1.613
12. Kevin Magnussen, Renault, +1.801
13. Felipe Nasr, Sauber, +1.831
14. Valtteri Bottas, Williams, +1.839
15. Daniil Kvyat, Toro Rosso, +1.844
16. Felipe Massa, Williams, +1.923
17. Jolyon Palmer, Renault, +2.097
18. Romain Grosjean, Haas, +2.196
19. Esteban Ocon, Manor, +2.728
20. Esteban Gutierrez, Haas, +2.756
21. Marcus Ericsson, Sauber, +2.861
22. Pascal Wehrlein, Manor, +3.773