Lewis Hamilton set the fastest time ahead of McLaren team-mate Jenson Button in a wet practice for the Korean Grand Prix.
Red Bull's Sebastian Vettel was third as he made a low-key return five days after winning a second world title.
Ferrari's Fernando Alonso chose not to set a time in first practice but finished fourth in the afternoon.
The only accident came when Mercedes' Nico Rosberg hit Jamie Alguersuari's Toro Rosso coming out of the pits.
The pit exit lane is around the outside of Turn One and the drivers' view of the track is obscured.
"It was not great circumstances," Rosberg said. "It just all came together. He came out at the exact wrong moment.
"I thought he was going to stop a little bit more to let me go through, but then he didn't and by the time I realised that, it was too late. It was just unfortunate."
Hamilton added: "When you leave the pits, you can't see people going into Turn One, you just take your normal line and if someone's going round that corner you can end up colliding with them unless they see you and there's not a lot of time for them to see you either. So it's definitely not a good pit lane."
Following the drivers' briefing with race director Charlie Whiting on Friday, it has now been decided that lights will be displayed to warn drivers on the track and leaving the pits when others cars are nearby.
Rosberg was not punished for his error, but Mercedes were fined a $10,000 (£6,300) for failing to appear before the stewards in good time.
At the last race in Japan, the leading trio of Button, Alonso and Vettel were separated by just two seconds but the rain made comparisons in Korea difficult.
Hamilton finished 0.104 seconds ahead of Button and took encouragement from ending the day as the fastest man.
"I think we are quick, the car feels good and Jenson showed in the last race that we can be competitive," said Hamilton, who has not been on the podium since winning in Germany in July.
"I have no doubts that we can challenge Red Bull but whether or not we can out-qualify them, which hasn't been done all year, will be interesting.
"It's early days and it was miserable today and irrelevant in terms of set-up. Tomorrow we'll get a better feel for where the car is."
Button ventured out for a late run on the soft tyres but told his team on the pit-to-car radio "You just can't get any heat into the tyres."
"It's so, so difficult out there today," said Button, who won the last race in Japan. "We were hoping the track would dry out but it didn't and there was no grip.
"We didn't do any high-fuel running, which we normally do, and I haven't got a clue what the car is going to be like in the dry.
"It would be nice to have the same gap to the cars behind. P3 will be an important session.It would nice to knock Red Bull off pole, we'll give it a go."
Neither Vettel nor his team-mate Mark Webber, who was fifth, felt the track was dry enough to test their limits on the soft tyres with Button and Alonso the only frontrunners to attempt late runs.
Both Red Bull drivers said they had a lot to learn about how the tyres would behave in the final practice session on Saturday before qualifying at 0600 BST.
Ferrari are also fighting for the win this weekend and spent the day trying to harvest as much data as they could despite the wet conditions.
Alonso, who aired his frustration about the wet weather on the pit-to-car radio, was almost two seconds off Button's time with team-mate Felipe Massa seventh.
"Here is a track that because of the [wet] conditions last year we don't have experience in the dry, especially in the race," said the Spaniard, who won the inaugural Korean race after the start was delayed by rain.
"Today we were not very happy or very competitive in wet conditions unfortunately. McLaren were very dominant, they did their times straight away.
"We take maybe too many laps to get the right tyre temperature and in that lap the tyre is not in good shape anymore, but maybe in the race this can be an advantage as we can be consistent."
Alguersuari did well to set the sixth fastest time for Toro Rosso while team-mate Sebastian Buemi also finished inside the top ten.
Scot Paul di Resta said he would take an "aggressive" approach in Korea and set the 10th fastest time.
It was a damp debut in first practice for Jean-Eric Vergne, who was given Alguersuari's Toro Rosso seat as Red Bull attempt to assess their junior drivers.
"I don't think the main thing is to compare myself against the other drivers today but to gain some experience for the future," said Vergne, who was four seconds off team-mate Sebastien Buemi.
Narain Karthikeyan and Karun Chandhok tuned up for their cameos at the Indian Grand Prix with final runs for HRT and Team Lotus.
"I wanted to drive before India so at least I got some time in the car," said Karthikeyan, who clocked 14 laps before flying on to Delhi.
Chandhok was a solid 11th fastest in the Lotus, prompting the team's chief technical officer Mike Gascoyne to say: "Good job by Karun in FP1 on a track he didn't know."