Toro Rosso boss Franz Tost says the performances of Formula 1 rookies Max Verstappen and Carlos Sainz Jr were the "surprisingly good" in 2015.
Verstappen finished as top rookie in 12th in the drivers' championship with fourth places in Hungary and the United States his highlights.
Sainz, meanwhile, ended the year 15th with 18 points, 31 adrift of Verstappen but suffered seven retirements compared to his team-mate's four.
Despite the contrasting fortunes and interest generated by Verstappen, Tost says both drivers impressed him.
"The performance from both drivers was surprisingly good," Tost told Autosport.
"Both drivers have done a really good job because they are new in Formula 1.
"With a more reliable car I think both would have surprised a bit more and have scored more points.
"I must say that the drivers have been the strongest part of the team this year.
"From pure speed, both drivers are quite similar."
Tost picked out Verstappen's overtaking ability as something that impressed him the most over the year.
"Knowing how far behind from the power-side we were this season, his overtaking moves that he showed, especially in Austin and Sao Paulo, were really impressive," added Tost.
"What impressed me most was that there was no action where you thought he did not have it under control.
"There was no locking fronts, no touching wheels with others in an unfair way.
"What also was impressive is he pulled back when he saw it didn't work. This control and the possibility to estimate whether you can start the pass or not is really good."
With Sainz, Tost praised the Spaniard's technical understanding and the way he dealt with poor reliability.
"His understanding of how to set up the car is impressive," said Tost. "Unfortunately, all the time he had technical failure.
"It was very important that he is mentally strong and that he can forget this, take it on the side and say 'OK, the race was bad because of the reliability issue so let's concentrate on the next race'.
"I'm convinced he will have a lot of highlights next year because sooner or later he will get back what he deserves."