Tost: Hartley still in the running for 2019, but needs to do more

For the various contenders of the second Toro Rosso seat alongside Daniil Kvyat next year, the chips should fall after the close of the 2018 season.

The 24-year-old Russian will make his return to F1 in 2019 after over a year on the sidelines, while Brendon Hartley and F2 racer Alexander Albon are the most likely candidates to race alongside Kvyat.

Hartley has been pulling out the stops lately in a bid to save his drive, but team boss Franz Tost said the Kiwi still hasn't convinced him to keep him onboard for a second season.

• Gasly hit by another engine grid penalty in Mexico

"He showed a good race in Austin. But nevertheless, if you look at the results, he has four points, Pierre has 28 points," Tost said.

"That means he has to improve his performance if he wants to stay in the team."

Albon, a former member of Red Bull's junior programme, was approached by the energy drink company's motorsport boss Helmut Marko, but his contractual situation with Nissan, for whom he is to race in Formula E, would require a full release, and negotiations are still ongoing.

In any case, a final resolution is unlikely until after the last race of the season at Yas Marina.

"First of all, you have to finish the season to get a clear picture," Tost told Crash.net.

"Then I assume in December, Red Bull will decide the driver line-up of Toro Rosso."

Gallery: The beautiful wives and girlfriends of F1 drivers

Keep up to date with all the F1 news via Facebook and Twitter