Max Verstappen's new power unit also caught Toto Wolff's eye

"When it comes to Red Bull, I think they've done a good job," Wolff told a select group of media, including GPblog.

For the first time in the team's history, Max Verstappen and Isack Hadjar's team is competing with an in-house engine, built in collaboration with Ford. They will go head-to-head against established manufacturers such as Wolff's Mercedes this year.

The Mercedes team boss added: "Hadjar did 107 laps I believe on the first day and was running reliably, so you have got to give them that."

In general, Wolff remained cautious when looking ahead to the upcoming season. He concluded: "The rest we will see when the stopwatch actually comes out."

The British driver also praised Red Bull Ford and Ferrari after the first pre-season test days of the season at the Circuit de Barcelona-Catalunya.

He said: "I think on the power unit side there's some impressive things from some of our competitors and that's quite surprising to be honest."