New Zealand's Brendon Hartley posted the eighth fastest time on the first day of Formula 1's first winter test in Barcelona.
In an encouraging sign for Toro Rosso Hartley ran 93 laps in his new Honda-powered car and set faster times than the Williams of Lance Stroll and Sergey Sirotkin, the Haas of Romain Grosjean, Force India's Nikita Mazepin and the Sauber of Marcus Ericsson.
Hartley was five one-hundredths of a second slower than 2017 champion Lewis Hamilton.
He ran 72 laps in the morning session – the most of any driver – before afternoon rain affected the latter part of the day and restricted teams.
"It was a really positive day," Hartley said.
"We had no issues at all but we stopped early because of the weather, it was almost icy conditions out there. Everything felt pretty good from out of the box.
"We had some test items that we didn't get to complete because of the weather, but getting through 93 laps on the first day of testing was a great start to the campaign with the new Toro Rosso-Honda (partnership).
"Balance-wise, we were pretty comfortable straight away but I still think there's potential to unlock. I'm really happy with how the day went, I think everyone probably struggled with tyres, but doing one corner slow the tyres drop out of the window and it was almost like an ice-skating rink."
Red Bull's Daniel Ricciardo set the fastest time of the day, 0.17s faster than the Mercedes of Valtteri Bottas. Kimi Raikkonen was third fastest in the Ferrari, ahead of Nico Hulkenberg's Renault and Fernando Alonso in the McLaren.
Alonso had an entertaining day. He recorded a purple sector (fastest time of the day) before losing control of his McLaren and bringing out a red flag after being caught in the gravel.
Toro Rosso boss Franz Tost was pleased with how the Honda-powered Toro Rosso performed on the first day.
"I don't see any problems, the power unit is working quite well and I am convinced Honda has the capacity and the know-how to develop the engine from to a level that we will be there, where we expect to be and this is how it is," Tost said.
"I see it as very, very positive because Honda have made a lot of progress in the last months on the reliability side as well as the performance side, so we are looking forward to the season."
Hartley's teammate Pierre Gasly will run the car on day two overnight before Hartley jumps back in the drivers' seat on day three.
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