In Austin, Toro Rosso drafted in Porsche World Endurance Championship driver Hartley to replace Gasly so the Frenchman could try to win the Super Formula title in Japan.

Meanwhile, Daniil Kvyat returned to the line-up after an absence of two race weekends in place of Renault-bound Carlos Sainz.

The team then announced Hartley would keep his seat for Mexico, with Gasly returning in place of Kvyat.

Motorsport.com understands the line-up will remain unchanged for the last two races in Brazil and Abu Dhabi as the team looks to finalise its 2018 driver plans.

Gasly, who missed out on the Super Formula title after the finale was cancelled because of the threat of a super typhoon, is expected to get the seat full-time next season after impressing in Malaysia and Japan.

It is believed Kvyat has dropped behind Hartley in the race to claim the second seat in 2018, with Toro Rosso and Red Bull keen to use the next three races to confirm recruiting him full-time is the right call.

Gasly and Hartley, who finished his debut grand prix 13th in Austin, remained coy on their futures when speaking to reporters in Mexico City on Thursday.

"There is not much I can comment on," said Hartley. "I'm pretty focused on this weekend, putting everything I learned from Austin into practice.

"There's going to be a bit more pressure, people will be expecting a bit more, but I'm feeling optimistic and confident I can do the job."

Gasly added: "It doesn't change what will happen this weekend. This is my focus at the moment."