Kimi Raikkonen at speed under the floodlights during the second free practice for the Singapore Grand Prix. Picture: Kim Hong-Ji / Reuters

Singapore - Kimi Raikkonen set the fastest time in practice for the Singapore Grand Prix on Friday but his Ferrari team-mate Sebastian Vettel suffered a frustrating start to the weekend in his bid to overhaul championship leader Lewis Hamilton.

Hamilton opened a 30-point cushion over Vettel with a surprise win in Monza two weeks ago and while Ferrari is regarded as having a superior car, Hamilton posted the second fastest time of the day in his Mercedes, just 0.011s off the pace.

Vettel has been criticised for dropping points due to some poor tactical driving this season but there was little he could do looking on from the Ferrari garage after his car suffered a radiator fluid leak midway through the second practice session.

Raikkonen lapped the 5.063 kilometre Marina Bay Street Circuit in 1m38.699s, with Hamilton just behind as the rejuvenated Red Bulls of Max Verstappen and Daniel Ricciardo finished third and fourth respectively, half a second further back.

Vettel's troubles began during the frantic middle sector of the second session as drivers looked to set their fastest laps on the high-grip hypersoft tyres before switching to slower compounds for race simulations. He was on course to challenge Raikkonen and Hamilton's best times before he clipped the wall on the final bend, causing him to abort his lap and dart into the pits with a fluid leak. He sat out the remainder of the session.

Valtteri Bottas was fifth fastest in the second Mercedes, ahead of Renault's Carlos Sainz, Haas' Romain Grosjean, McLaren's Fernando Alonso and Vettel down in ninth place after completing just 12 laps.

Red Bull joy

In the first session of the day, Red Bull enjoyed a welcome return to the top of the timesheets after a few quiet races with Ricciardo and Verstappen securing a one-two for the team in the baking afternoon heat.

The demanding 23-turn circuit has traditionally favoured the Red Bulls and Ferraris, so it was no surprise to see Vettel and Raikkonen clock the next quickest times as Mercedes opted to concentrate on long runs instead of qualifying simulations.

Ricciardo has failed to finish the past two Grands Prix since Formula One returned from its summer break but he again showed his street circuit prowess by clocking 1m39.711s in the scorching Singapore sun. Verstappen lapped in 1m39.912s to edge out Vettel by 0.085s but Vettel will be looking for an improved third practice session and qualification on Saturday as he seeks to rein in Hamilton with seven rounds remaining.

"We can recover most of it tomorrow so we are not that worried," Vettel said. "It looks like being close but we won't really know until Sunday."

Sauber's Charles Leclerc continued to show the sort of form that encouraged Ferrari to sign him as Raikkonen's replacement for next season by finishing ninth in an opening session that ended on a sour note when he crashed into a wall at Turn 13.

The 20-year-old apologised to his team on the radio as he left his car stranded in an escape road with two minutes left in the session, a sight that will disappoint his future employers, who will hope he does not make similar mistakes next season.

RESULTS - First Session

1 Daniel Ricciardo (Red Bull) - 1m39.711s

2 Max Verstappen (Red Bull) +0.201s

3 Sebastian Vettel (Ferrari) +0.286

4 Kimi Raikkonen (Ferrari) +0.775

5 Nico Hulkenberg (Renault) +1.394

6 Lewis Hamilton (Mercedes) +1.521

7 Carlos Sainz (Renault) +1.618

8 Valtteri Bottas (Mercedes) +1.718

9 Charles Leclerc (Sauber) +2.324

10Romain Grosjean (Haas) +2.397

11 Marcus Ericsson (Sauber) +2.697

12 Sergio Perez (Force India) +2.701

13 Kevin Magnussen (Haas) +2.741

14 Fernando Alonso McLaren +2.919

15 Esteban Ocon Force India +3.466

16 Pierre Gasly Toro Rosso +3.529

17 Brendon Hartley Toro Rosso +3.774

18 Lance Stroll Williams +4.138

19 Sergey Sirotkin Williams +4.325

20 Stoffel Vandoorne McLaren +5.449

Second Session

1 Kimi Raikkonen (Ferrari) 1m38.699s

2 Lewis Hamilton (Mercedes) +0.011s

3 Max Verstappen (Red Bull) +0.522

4 Daniel Ricciardo (Red Bull) +0.610

5 Valtteri Bottas (Mercedes) +0.669

6 Carlos Sainz (Renault) +1.575

7 Romain Grosjean (Haas) +1.685

8 Fernando Alonso (McLaren) +1.760

9 Sebastian Vettel (Ferrari) +1.934

10 Nico Hulkenberg (Renault) +1.969

11 Sergio Perez (Force India) +2.075

12 Marcus Ericsson (Sauber) +2.113

13 Esteban Ocon (Force India) +2.171

14 Charles Leclerc (Sauber) +2.363

15 Kevin Magnussen (Haas) +2.455

16 Stoffel Vandoorne (McLaren) +2.465

17 Brendon Hartley (Toro Rosso) +2.843

18 Pierre Gasly (Toro Rosso) +2.916

19 Lance Stroll (Williams) +3.442

20 Sergey Sirotkin (Williams) +3.482