A switch to slick tyres in the closing stages of a drying second Friday practice session for the Malaysian Grand Prix saw an all-CRT top four at Sepang.

After a close morning practice in which Dani Pedrosa, Casey Stoner and Jorge Lorenzo were covered by just 0.3s over the two-minute lap, storm clouds and the inevitable rain arrived just before the start of FP2.

The rain soon passed, but parts of the circuit - especially turns one and two - were soaked, with the first MotoGP bikes on track throwing up large 'rooster tails' of spray.

With Sunday's MotoGP race starting at a later than normal 4pm, increasing the thunderstorm risk, many of the 21 riders made an effort to at least test their wet set-ups.

However Pedrosa, Lorenzo and Stoner were among eight riders not to record a lap time this afternoon.

Gresini Honda's Alvaro Bautista was quickest from Ducati riders' Valentino Rossi and Nicky Hayden by the time most had abandoned rain tyres, after 15 of the 45 minutes.

With further dark clouds gathering, a lull followed as competitors waited for conditions to either get worse or better.

Ultimately, further rain stayed away, with Avintia's Ivan Silva the first to try slicks with 14 minutes to go, while Rossi returned on wets soon after - perhaps to understand how long the wets could endure such conditions should similar on-off weather occur in the race.

The session ended with Silva fastest by 1.4s from fellow slick CRT riders Michele Pirro (Gresini), Randy de Puniet (Aspar) and Roberto Rolfo (Speed Master).

Cardion AB Ducati's Karel Abraham was the only manufacturer rider on track at the end of the session.

Bautista and Rossi, who took his most recent MotoGP win at Sepang in 2010, were pushed down to sixth and seventh by the late improvements.

Friday practice (2):

1. Ivan Silva
2. Michele Pirro
3. Randy De Puniet
4. Roberto Rolfo
5. Karel Abraham
6. Alvaro Bautista
7. Valentino Rossi
8. Aleix Espargaro
9. Nicky Hayden
10. James Ellison
11. Stefan Bradl
12. Hector Barbera
13. Colin Edwards
Casey Stoner
Andrea Dovizioso
Danilo Petrucci
Ben Spies
Dani Pedrosa
Cal Crutchlow
Yonny Hernandez
Jorge Lorenzo