Red Bull's Sebastian Vettel signalled his Formula One title defence isn't over on Friday, clocking the quickest times in both practice sessions for the Singapore Grand Prix.

On the floodlit street circuit, damp after heavy afternoon rain, the two-time defending champion recorded 1min 50.566sec in the first stint and then lowered it to 1:48.340 in the later run.

McLaren's Jenson Button was second quickest in the later session followed by the championship leader, Fernando Alonso of Ferrari. Red Bull's Mark Webber was fourth fastest with Lewis Hamilton placing fifth.

"This is a tough circuit for drivers and cars. Overall we can be happy with today, but let's see what we do tomorrow," Vettel said.

"Most important is that the car seemed to work on either tyre, so we go from there. The kerbs have been changed this year, which is an improvement."

Vettel, Formula One's youngest double champion, lies fourth in the season standings and 39 points back from pace-setter Alonso with seven races to go.

"I would say today was positive, even if the first impression is that we are not as competitive as we were in Monza, two weeks ago," Alonso said.

"We still lack a bit of performance on tracks like this one, where you need maximum aerodynamic downforce."

Alonso leads the standings by 37 points ahead of Hamilton, who has won two of the last three races. Lotus's Kimi Raikkonen and Vettel are both close behind with Webber fifth.

With Hamilton and Button sharing the last three race wins between them, the pressure is on Red Bull to fix reliability issues which have plagued Vettel and keep him back in the championship race.

Force India pair Paul di Resta and Nico Hulkenberg were sixth and seventh fastest in the later run, with Mercedes' Nico Rosberg, Ferrari driver Felipe Massa and Lotus pilot Romain Grosjean rounding out the top 10.

Ma Qinghua, who became the first Chinese driver to take part in Formula One during practice for this month's Italian Grand Prix, finished slowest of the opening practice for HRT.