Kiwi Brendon Hartley will start from the back of the grid in this morning's US Grand Prix but his future with the Toro Rosso team looks bright.

The 27-year-old was a late call-up for the event at the Circuit of the Americas, replacing youngster Pierre Gasly for the one-off race, but team boss Franz Tost gave the strongest hint yet that the Le Mans 24-Hour champion is in line for an extended stay with the team.

Not only is he likely to race at next weekend's Mexican Grand Prix but a fulltime drive next year is now on the cards.

"If he shows the performance in the next few races and improves then I am convinced that he will become a driver for next year," Tost told Radio Sport. Asked whether Hartley will back up next week in Mexico, Tost all but guaranteed the New Zealander would get more chances to impress and secure a fulltime drive.

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"I think so," he admitted. "We will have a conversation after Austin but it looks good he will drive for us in Mexico."

Gasly, who skipped today's race to contest the final round of the Asian Super Formula event in Japan, is almost certain to return to the team in Mexico leaving fulltime driver Daniil Kvyat vulnerable of missing out altogether.

While Kvyat out-qualified Hartley, he has struggled this season to the point where he was dropped for the past few Grands Prix while rookie Gasly was given an opportunity to establish himself in Formula 1.

Tost refused to comment on the Russian's future.

"Let's wait and see -- it is not decided yet. It is open and it will be a decision from Red Bull."

Hartley did a solid job in qualifying -- eventually setting the 18th fastest time in his first attempt in the category.

The New Zealander's practice pace has been praised and he has certainly not looked out of place in his debut weekend.

"Very good," was the assessment from Tost. "He improved his performance from lap to lap, he is getting more familiar with the team and in qualifying he showed a good performance because he must know it is so difficult in Formula 1 to get everything together in qualifying, understand the tyres, manage the tyres the right way, manage the traffic correctly -- it was very windy today and he sorted out all of that and came up with a respectful time."

The surprise call-up left Hartley with little opportunity to ready himself for the sport's biggest stage.

He hadn't driven a single-seater since 2012 and spent the past two days familiarising himself with the car and the environment.

"He couldn't do any test with the car -- we had to find the correct seat position for him because his helmet was moving a little bit too much because of the air flow in the cockpit -- all these little things take time," Tost added.

"I am convinced Brendon will put up a better performance in the next race he gets," said the team chief.

"We will see he is familiar with the car after two or three races. I am convinced he can do this -- he is very committed and will climb up."

As for Tost's expectations in the race?

He is hopeful Hartley's race pace will be more competitive than his one-lap speed in qualifying.

"He did quite good long runs [on Saturday] and [Friday]. I expect a good race from him.

"It is difficult to overtake here -- if he is around 12th or 13th I think that will be a great result."