Lotus driver Kimi Raikkonen narrowly edged out Red Bull's Sebastian Vettel to set the pace in second practice at the Malaysian Grand Prix.
Raikkonen, who won the opening race in Australia was just 0.019 seconds ahead, with the Ferraris of Felipe Massa and Fernando Alonso third and fourth.
Red Bull's Mark Webber was fifth from Lotus's Romain Grosjean.
Force India's Paul di Resta was eighth, Mercedes' Lewis Hamilton ninth and McLaren's Jenson Button 12th.
Raikkonen looked in impressive form throughout the day - as well as topping the second session, he was second in the first session, just 0.068secs behind Webber, who was ahead of Vettel, Alonso and Mercedes' Nico Rosberg.
"We didn't get as much done as we wanted to but that's the same for everyone," said Vettel of the expected rain during free practice two. "One part of the track was quite soaked and one part pretty dry.
"This afternoon we couldn't do that much [race preparation] because of the weather. It looked pretty OK but quite a mess if you look at how long the tyres last. There is some footage to look at too; it doesn't look very nice, I have to say.
"I hope we have enough tyres to survive the race. Tyre wear was pretty severe for everyone."
The leading runners appeared closely matched, with only 0.384secs separating Webber from Alonso in the first session and 0.416secs between Raikkonen and the Spaniard in the second.
Mercedes appear to be off the pace of the front-runners - Rosberg was 0.879secs slower than Raikkonen in the second session and Hamilton 1.005secs back.
"I just didn't get to do a single-lap time on either set of tyres because we went out on heavier fuel and then when we went back out it rained," said Hamilton.
"The race run felt pretty strong. It felt quick and the car felt better than in the last race. I think we have made a step forward with the car but it's difficult to judge where we are."
Williams development driver Susie Wolff, acting as a BBC analyst on Friday, said: "It is clear Raikkonen is on a roll this weekend - second in P1, fastest in P2 and fastest in the wet as well.
"Everything is where we expected from first practice. Everyone is in roughly the same order as Australia.
"Lotus are quick, as are Ferrari and Red Bull and I think it will come down to tyre management on Sunday.
"McLaren are going to have to stay worried. I don't think they have made much progress. In reality, with only five days since Melbourne there is not a lot they can do. But after this weekend they have three weeks before China to look at the data and make some improvements."
McLaren's Jenson Button was further hindered when he suffered two punctures on the same set of tyres.
Both sessions were run in typically hot, humid conditions, and increasingly heavy rain affected the second session from about the halfway mark until the drivers all returned to the pits with 35 minutes of the 90-minute session remaining.
The rain abated after a few minutes and drivers took to the track on treaded 'intermediate' tyres before squeezing in some laps on slicks in the closing minutes.
BBC F1 technical analyst Gary Anderson said: "It's an opportunity to see what the track will be like if it rains on Sunday. It's a chance to experiment with different lines and so on."
It was clear from the dry running that tyre usage could be a decisive factor in Malaysia, as it was in Australia.
All the teams struggled to one degree or another with 'graining', where the surface of the tyre overheats and shreds.
Frenchman Jules Bianchi continued his impressive form for Marussia at the start of his first full season.
Bianchi, a member of the Ferrari driver academy, was 18th fastest, ahead of the Williams of Finn Valtteri Bottas and 1.93secs quicker than team-mate Max Chilton. The Englishman was about half a second off Bianchi in the first session.
1. Kimi Raikkonen (Fin), Lotus - 1 minute, 36.569 seconds
2. Sebastian Vettel (Ger), Red Bull - 1:36.588
3. Felipe Massa (Brz), Ferrari - 1:36.661
4. Fernando Alonso (Spa), Ferrari - 1:36.985
5. Mark Webber (Aus), Red Bull - 1:37.026
6. Romain Grosjean (Fra), Lotus - 1:37.206
7. Nico Rosberg (Ger), Mercedes - 1:37.448
8. Paul di Resta (GB), Force India - 1:37.571
9. Lewis Hamilton (GB), Mercedes - 1:37.574
10. Adrian Sutil (Ger), Force India - 1:37.788
11. Sergio Perez (Mex), McLaren - 1.37.838
12. Jenson Button (GB), McLaren, 1:37.865
13. Nico Hulkenberg (Ger), Sauber, 1.38.068
14. Esteban Gutierrez (Mex), Sauber, 1.38.645
15. Jean-Eric Vergne (Fra), Toro Rosso - 1.38.738
16. Pastor Maldonado (Ven), Williams - 1:38.801
17. Daniel Ricciardo (Aus), Toro Rosso - 1.38.904
18. Jules Bianchi (Fra), Marussia, 1.39.508
19. Valtteri Bottas (Fin), Williams - 1.39.660
20. Charles Pic (Fra), Caterham - 1.40.757
21. Giedo van der Garde (Ned), Caterham - 1.40.768
22. Max Chilton (GB), Marussia - 1.41.438