Jorge Martin set the pace on the opening day of practice for the Moto3 class in the United States, with championship leader Aron Canet 0.283 seconds adrift in second.
Modifications to certain points of the 3.43-mile Circuit of the Americas in a bid to reduce some of the bumps mean the track was incredibly dusty for the opening Moto3 session of the day.
With each lap cleaning up the circuit, top spot constantly changed hands over the course of the 40-minute session, with Enea Bastianini, former CoTA poleman Philipp Oettl, Darryn Binder, Jaume Masia, Marco Bezzecchi and Martin all taking turns leading the pack.
Rookie Masia shot to the top of the standings with just under five minutes remaining with a 2:20.424s, though was deposed by Bezzecchi just moments later by two tenths of a second.
On his final effort, Martin edged a further two tenths ahead of the Prustel GP rider to cement top spot with a 2:20.005s, with Bastianini and Andrea Migno shuffling Bezzecchi back to fourth, ahead of Canet.
The track had improved significantly for the final Moto3 outing of Friday, with Ayumu Sasaki going top with the best lap of the day immediately with a 2:19.784s.
Lorenzo Dalla Porta hit top spot soon after with a 2:19.391s, but was usurped by Niccolo Antonelli by just over a tenth. A further tenth allowed 2017 Austin poleman Canet to take over at the head of the field, though only for 30 seconds as Bastianini crept ahead with a 2:19.003s.
A follow-up effort of 2:18.339s was beaten by Martin's 2:18.319s – which was cancelled and then reinstated – with 12 minutes remaining. However, just as Martin's lap was reconfirmed, Bastianini returned to top spot with a 2:18.021s.
Laptimes continued to tumble to the chequered flag, with Martin deposing Canet at the death with a 2:17.423s, with his Gresini teammate Fabio Di Giannantonio completing the top three.
Bezzecchi ended Friday fourth overall from the Leopard pair of Dalla Porta and Bastianini, with Oettl heading Antonelli, Livio Loi and Sasaki to round out the top 10.
Nieto's Andrea Migno was the only crasher on Friday, the Italian suffering a fast fall at Turn 4 in the closing stages after hitting a bump on the way into the corner. Migno was unscathed and finished up in 17th.
