Brendon Hartley is among the candidates in contention to drive for Scuderia Toro Rosso in the United States Grand Prix, with the possibility of a full-time call-up next year.
While Daniil Kvyat will race one STR car at Austin, the other is currently vacant due to Super Formula title contender Pierre Gasly's participation in that championship's season finale at Suzuka on the same weekend.
According to Autosport, Hartley could fill the other seat, with Sebastian Buemi the back-up option.
STR team principal Franz Tost is believed to want to trial a potential new driver in the USA.
Gasly is favourite to get one of the seats, having impressed in his appearances in Malaysia and Japan for the team, but Tost and Red Bull are less certain about the second seat.
Hartley has emerged as the frontrunner to drive at Austin, with Tost believed to be open to the possibility of the New Zealander racing for the team next year.
The 27-year-old held the role of Red Bull's official reserve driver in 2009 and 2010 and completed one test for RBR in 2008 and two for STR in 2009 before being dropped from Red Bull's junior programme.
Hartley also took part in a young driver test at Magny-Cours for Mercedes in 2012, as well as spending time in the team's simulator.
He has enough superlicence points to race in F1 but is currently contracted to Porsche and competing in the World Endurance Championship, where he could win the title this weekend at Fuji.
In addition, it is also believed he is close to a deal to race for Chip Ganassi in IndyCar next year, meaning there are contractual issues to be ironed out before his Austin appearance can be confirmed.
With that in mind, STR and Red Bull have lined up Buemi, who raced for Red Bull's junior team between 2009 and 2011, to step in if a deal for Hartley cannot be agreed.
The United States Grand Prix takes place on October 20-22.