MONTMELO, SPAIN -- Lewis Hamilton topped the fourth day of pre-season testing as he finally notched up a decent day of mileage in the new Mercedes W09.
The freezing conditions of the first three days finally gave way to warmer temperatures on Thursday morning, but fog and drizzle left the track soaked for the start of the session.
Due to the cold weather, Hamilton had completed just 25 laps prior to Thursday but made up for lost time on a drying track in the afternoon by adding a further 69 to his total for the first test. Teammate Valtteri Bottas had braved the worst of the conditions in the morning and added a further 60 laps to the 152 he had completed earlier in the week.
Hamilton set the fastest time -- a 1:19.333 -- using the medium compound tyre on a dry track, giving him the fastest time of the week and leaving him half a second clear of Stoffel Vandoorne in the McLaren. Vandoorne completed the majority of his 110 laps in the wet, but McLaren bolted a set of hyper-soft tyres -- Pirelli's softest compound -- to his MCL33 in the afternoon to allow him to set a personal best of 1:19.854. Vandoorne handed the McLaren to teammate Fernando Alonso at 15:00, who added another 51 laps to the team's tally by the end of the day. It was an important day for McLaren after missing out on dry running earlier in the week due to separate wheelnut and exhaust issues.
Sebastian Vettel was third fastest as he returned to the cockpit of his new Ferrrari and completed 120 laps. His fastest time was set on soft tyres and was 0.908s off Hamilton's best.
"It's a new car, so everybody's excited," Vettel said when asked for his first impressions of the car. "It's just a bit of a shame when you don't run as much as you can. But I think it's the same up and down the pit lane. Everyone is keen to get some running.
"It's always tricky when everyone is so excited, they have a new car, and you're not able to tick all the boxes and can't find an answer to all the questions that you have."
Kevin Magnussen used super-soft tyres to set the fourth fastest time and was just 0.076s shy of Vettel by the end of the day with a total of 92 laps in his Haas.
Elsewhere, Red Bull struggled for mileage after opting against running in the wet conditions in the early morning. When Max Verstappen finally took to a drying track in the afternoon, the RB14 suffered a mechanical issue and had to return to the pits after just a couple of runs. With the problem fixed soon after, he returned to a fully dry track but ended up in the gravel trap at Turn 12 as he was on his personal best lap. After being recovered to the garage, the car spent the rest of the day in the pits.
Verstappen's off was one of three red flags during the day after Marcus Ericsson put his Sauber in the gravel at Turn 2 in the morning and Charles Leclerc, who took over driving duties at Sauber in the afternoon, caused a red flag when he ran wide at Turn 3.
Sergio Perez completed his first miles in the Force India after missing out on his scheduled running due to snow on Wednesday. However, he only completed 65 laps in total, leaving Force India with the least mileage of all ten teams at the end of the first test with just 166 laps. At the other end of the spectrum, Pierre Gasly completed 147 laps in the Honda-powered Toro Rosso. In terms of laps, it was the Japanese manufacturer's most successful test day since returning to the sport in 2015 and means Toro Rosso was only beaten for mileage on Thursday by McLaren.
Renault and Williams had solid days and completed over 100 laps each as they both split their running between their two race drivers. Carlos Sainz and Nico Hulkenberg completed a total of 119 laps in the R.S.18 and Lance Stroll and Sergey Sirotkin managed 101 laps in the FW41.
Times at close:
1. Hamilton, Mercedes, 1:19.333, 69 laps (Medium)
2. Vandoorne, McLaren, 1:19.854, 110 laps (Hyper-soft)
3. Vettel, Ferrari, 1:20.241, 120 laps (Soft)
4. Magnussen, Haas, 1:20.317, 96 laps laps (Super-soft)
5. Alonso, McLaren, 1:20.929, 51 laps (Super-soft)
6. Sainz, Renault, 1:20.940, 60 laps (Medium)
7. Stroll, Williams, 1:21.142, 54 laps (Soft)
8. Perez, Force India, 1:21.973, 65 laps (Soft)
9. Verstappen, Red Bull, 1:22.058, 35 laps (Soft)
10. Gasly, Toro Rosso, 1:22.134, 147 laps (Super-soft)
11. Hulkenberg, Renault, 1:22.507, 49 laps (Soft)
12. Bottas, Mercedes, 1:22.789, 60 laps (Medium)
13. Leclerc, Sauber, 1:22.808, 59 laps (Soft)
14. Ericsson, Sauber, 1:23.825, 79 laps (Super-soft)
15. Sirotkin, Williams, 1:31.979, 47 laps (Intermediate)