Maverick Vinales pipped Marc Marquez to top MotoGP Americas Grand Prix FP2, while both works Ducatis missed the provisional Q2 places.
Vinales shadowed Marquez in first practice, and snatched top spot late on with a 2:03.857s to lead the Honda rider – who tested HRC's version of the controversial Ducati tyre cooler – by 0.044 seconds.
Alex Rins set the initial pace on the Suzuki with a 2:06.052s lap in the opening moments, ending Yamaha's Valentino Rossi's short-lived tenure at the top of the pile.
FP1 pacesetter Marquez soon picked up where he left off in the morning session, guiding his Honda to a 2:05.276s, before firing in the first sub-2:05s lap of the weekend with a 2:04.719s.
Marquez put himself four tenths out of reach of Pramac's Jack Miller, though Vinales usurped the Australian with 21 minutes remaining and put himself just over two tenths outside of Marquez's effort.
Times remained largely stagnant through to the final 10 minutes, with LCR's Cal Crutchlow surviving a big moment rounding Turn 18 to put his Honda up to second with a 2:04.920s.
Rossi lifted himself out of the fringes of the top 10 into second moments after Crutchlow, beating the Briton's time by a tenth to become Marquez's closest challenger.
With three minutes to go, Miller stalked Marquez on a flying lap to deny the Honda rider top spot with a 2:04.005s, while KTM's Pol Espargaro used Dovizioso as a reference to leap up to third.
Vinales, on his own, ensured Pramac would not remain pacesetters on his final lap, firing in a 2:03.857s to end Friday's running fastest of all.
Rossi leaped up to third late on with a 2:04.003s, edging ahead of Miller by 0.002s. Crutchlow completed the top five from Rins and Espargaro.
Franco Morbidelli headed Sepang Racing team-mate Fabio Quartararo to end the day eighth overall, while Francesco Bagnaia rounded out the top 10 – denying factory Ducati counterpart Dovizioso by just under a tenth.
Rins' improvement to sixth in the dying stages forced Dovizioso out, with Joan Mir [Suzuki] and the sister works Ducati of Danilo Petrucci trailing him.
With rain forecast for Saturday's third practice, both factory Ducati riders, along with Honda's Jorge Lorenzo in 16th, may already be consigned to contesting the first part of qualifying.
