Porsche topped the timesheets during Thursday practice for this weekend's Austin round of the World Endurance Championship.
Earl Bamber and Nick Tandy posted the quickest times of each 919 Hybrid during the opening 90-minute session at Circuit of the Americas, before Neel Jani came through to take the overall top spot in the afternoon.
Jani, who shares the #1 Porsche with Tandy and Andre Lotterer, posted a time of 1:45.860s to give that car the edge heading into Saturday's qualifying run.
Bamber went on to improve on his morning marker with a 1:46.401s lap to back up Jani, which put both Porsches ahead of the rival Toyota TS050 Hybrids.
Toyota's nearest challenge came courtesy of Kamui Kobayashi, who finished 0.959 seconds shy of Jani's benchmark in the #7 machine.
Sebastien Buemi lapped fourth quickest in the sister car, more than two seconds adrift.
In the LMP2 category, Vaillante Rebellion echoed Porsche with a one-two of its own, orchestrated by Bruno Senna and Mathias Beche.
Senna, driving the 6 Hours of Mexico-winning ORECA co-driven with Nicolas Prost and Julien Canal, registered a best lap of 1:54.601s to pip his stable-mate in the other car by a tenth.
Alex Lynn went third fastest for G-Drive Racing, while Nicolas Lapierre lapped fourth in the Singnatech Alpine entry.
Friday practice in GTE-Pro was led by the #95 Aston Martin, which had Nicki Thiim behind the wheel for its table-topping 2:04.023s effort.
That was enough to marginally out-pace the pairs of Fords, Porsches and Ferraris, which occupied second through to seventh.
Aston Martin also led the way in GTE-Am, as Pedro Lamy topped the field by 1.4 seconds.
Qualifying for round six of the World Endurance Championship takes place on Friday, September 15th at 15:05 Central Time.
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