Mercedes’ Nico Rosberg topped the timesheet in the opening practice session for Sunday’s Malaysian Grand Prix but it wasn’t plain sailing for the silver arrows in Sepang as Lewis Hamilton was sidelined early in the session by a power unit problem.

With just half an hour on the clock and with only four laps to his name, Hamilton steered his Mercedes off track at Turn 9 and stopped, radioing his team to explain that “something happened, I lost power”.

“There was a click in the rear, the gears still worked, I just pulled over and stopped like you told me to. I would have made it back,” he added.

Mercedes engineers, however, told the championship leader that they were concerned about the issue as the power unit in Hamilton’s car was a race engine.

The Briton’s car was brought back to the pits on the back of a truck and he took no further part in the session.

On the other side of the garage Rosberg had a more profitable outing. The German was fastest throughout the session, initially by almost a second and then, with half and hour remaining, improving to almost 1.5s ahead of Lotus’ Romain Grosjean.

Ferrari’s Kimi Raikkonen then went quicker than the French driver and eventually ended the session just under four tenths down on Rosberg’s best time of 1:40.124 with a late blast in the final minutes of the session.

Third place was taken by the Finn’s team-mate Sebastian Vettel. With extensive work being done on Vettel’s car the four-time champion didn’t set a time in the session until almost an hour had elapsed. He eventually climbed to third, however, to finish with a best lap of 1:40.985, eight tenths shy of Rosberg’s P1 time,

After running second to Rosberg at the hour mark, Grosjean dropped to fourth, just over three tenths of a second behind Vettel. Fifth was the impressive Carlos Sainz who powered his Toro Rosso to within 1.5s of Rosberg and almost two tenths of a second ahead of Red Bull Racing’s Daniel Ricciardo. Sainz had also been second quickest to Rosberg for much of the opening phase of the session.

Max Verstappen was seventh in the second Toro Rosso ahead of Williams’ Valtteri Bottas, who was making his return to the cockpit after suffering a back injury in qualifying in Melbourne that kept him out of the season-opening Australian Grand Prix.

The Finn finished his first session 1.758 down on the morning’s P1 time but ahead of the second Red Bull of Daniil Kvyat and the Sauber of Marcus Ericsson.

At McLaren, Fernando Alonso made his long-awaited reappearance following the testing accident in February that kept him out of the season’s first race.

The Spaniard passed FIA medical checks on Thursday in Sepang and in his first session back completed 20 laps at the wheel of McLaren’s MP4-30 to claim 14th place on the timesheet, five hundredths of a second behind F1 weekend debutant and Ferrari Academy driver Raffaele Marciello, who took over Felipe Nasr’s Sauber for the session.

Alonso’s time put him ahead of the Force India cars of Nico Hulkenberg and Sergio Perez, with Jenson Button 17th in the second McLaren.

After failing to take to the track in Australia two weeks ago, the Manor Marussia team made its race weekend debut in Malasysia with Will Stevens completing eight laps of the Sepang Circuit for a best lap of 1:46.686. Team-mate Roberto Mehri also managed eight laps, with his best lap putting him 7.5s behind Rosberg, a result that would in qualifying put himself beyond the 107 per cent of the fastest time needed to race.

2015 Malaysian Grand Prix – Free Practice 1
1 Nico Rosberg Mercedes 1:40.124s – 20
2 Kimi Raikkonen Ferrari 1:40.497s 0.373s 17
3 Sebastian Vettel Ferrari 1:40.985s 0.861s 13
4 Romain Grosjean Lotus 1:41.543s 1.419s 14
5 Carlos Sainz Toro Rosso 1:41.596s 1.472s 26
6 Daniel Ricciardo Red Bull 1:41.787s 1.663s 15
7 Max Verstappen Toro Rosso 1:41.803s 1.679s 23
8 Valtteri Bottas Williams 1:41.882s 1.758s 23
9 Daniil Kvyat Red Bull 1:42.055s 1.931s 18
10 Marcus Ericsson Sauber 1:42.064s 1.940s 16
11 Felipe Massa Williams 1:42.103s 1.979s 23
12 Pastor Maldonado Lotus 1:42.567s 2.443s 19
13 Raffaele Marciello Sauber 1:42.621s 2.497s 13
14 Fernando Alonso McLaren 1:42.885s 2.761s 20
15 Nico Hulkenberg Force India 1:42.893s 2.769s 13
16 Sergio Perez Force India 1:43.054s 2.930s 15
17 Jenson Button McLaren 1:43.100s 2.976s 19
18 Will Stevens Marussia 1:46.686s 6.562s 8
19 Roberto Merhi Marussia 1:47.683s 7.559s 8
20 Lewis Hamilton Mercedes - - 4