Lorenzo Dalla Porta edged Jorge Martin to top spot by just 0.074 seconds in final Moto3 practice ahead of qualifying at the Circuit of the Americas.
Gresini's Martin topped both of Friday's sessions, and made a play to complete the hat-trick in the final seven minutes with a 2:17.778s.
The Spaniard improved to a 2:17.573s on his follow-up tour, going six-tenths clear of Gresini teammate Fabio Di Giannantonio in second spot.
Martin looked good to hold onto first place, but Leopard's Dalla Porta fired in a 2:17.499s in the dying seconds to snatch it away from the Qatar race winner, who was unable to retaliate after encountering heavy traffic on his final lap.
However, Martin still remains the pacesetter on the combined timesheets thanks to his FP2-topping time of 2:17.423s.
Di Giannantonio completed the top three, with the second Leopard bike of Enea Bastianini four hundredths further adrift in fourth spot ahead of last year's Austin poleman and current championship leader Aron Canet.
Marco Bezzecchi led the way just as the session counted down into its final quarter, the Argentina race winner deposing Juame Masia with a 2:18.310s.
Bezzecchi was able to improve to a 2:17.882s on his Prustel GP KTM, but that was only good enough for sixth at the chequered flag ahead of RBA's Gabriel Rodrigo.
Tatsuki Suzuki slotted into eighth in the end, with 2016 Austin polesitter Philipp Oettl two-thousandths behind in ninth.
Red Bull KTM Ajo's Darryn Binder made a strong start to proceedings, shooting to the top of the standings with a 2:19.437s in the opening stages.
Improving to a 2:18.398s in the closing minutes of third practice, Binder found himself demoted to 10th.
