Red Bull's Sebastian Vettel clocked the quickest lap of Friday practice for the Singapore Grand Prix as crashes interrupted each session at Marina Bay.

The German lapped in one minute 48.650 seconds in the second session to beat Renault's Fernando Alonso by 0.274secs.

Rubens Barrichello topped the first session ahead of his Brawn team-mate and championship leader Jenson Button.

Renault's Romain Grosjean and then Red Bull's Mark Webber crashed out on the dusty, slippery street circuit.

The first session was stopped for 10 minutes half-way through after Grosjean lost control of his Renault and crashed at Turn 17.

The Frenchman's mishap was laden with irony as it was at the same point where former Renault driver Nelson Piquet Jr was ordered to crash last year, the controversial race-fixing incident which saw Renault handed a suspended ban from F1 this week.

Webber, who was third fastest in the first session, was running aggressively at the top of the timesheets around the hour mark of the second when he crashed head on into a wall on the pit straight.

That meant that despite finishing sixth under the lights the Australian, who has faded somewhat from the championship battle in recent races, lost running time and only managed 14 laps.

"It was simply driver error," Red Bull team boss Christian Horner told BBC Sport.

Alonso, who won the now-infamous Singapore race in 2008, was fourth fastest in the first session.

Headline practice times are rarely a reliable guide to race form, but practice is important for teams as they attempt to discover their best set-up for qualifying and the race.

The Marina Bay street circuit, which hosted F1's first-ever night race last year, has also been slightly modified so drivers were getting used to the track adjustments.

Along with Grosjean's incident, Kovalainen and Force India's Adrian Sutil also spun in the first session but drivers gained more grip as more rubber went down on the track throughout the day.

The Brawn drivers were first and second in the first session, with Barrichello - who trails Button by 14 points in the drivers' championship with 40 available for the remainder of the season - 0.2secs ahead of the Englishman.

Like many teams coming into the 14th race of 2009, the Brawn cars were running with a moderately updated package - and it appeared to pay dividends earlier in the day for a team fresh from securing a one-two at the Italian GP.

"We had a pretty full-on evening with a busy programme of set-up evaluations and tyre testing," said Button.

"The car feels reasonable and with some more work later tonight, we should be ready for a good weekend."

Renault have a new management structure after the team parted company with boss Flavio Briatore and engineering director Pat Symonds in the wake of the race-fixing scandal.

Technical director Bob Bell has been promoted to acting team principal and he said it was important to move on from the scandal which had engulfed F1 in recent weeks as they returned to the scene of their wrongdoing.

"It's going to be an interesting weekend for the team," he said. "We're here to do the best job we can as normal, keep our heads down, and put everything that's happened in the past few weeks behind us.

"We've always tried to structure the team so that if people do disappear for whatever reason we can recover.

"Fernando likes this track, he's confident and I think he can do a good job here."

The third and final practice session starts at 1200 BST on Saturday before qualifying takes place at 1500 BST.