Mercedes driver Lewis Hamilton beat Red Bull's Sebastian Vettel to set the pace in second practice at the Korean Grand Prix.
Hamilton was 0.108 seconds ahead of the world champion, with the Englishman's team-mate Nico Rosberg third and the second Red Bull of Mark Webber fourth.
Ferrari's Felipe Massa took fifth place, split from team-mate Fernando Alonso by Lotus's Romain Grosjean.
'Happy' Hamilton enjoying Korea
Lotus's Kimi Raikkonen was eighth, ahead of McLaren's Jenson Button.
The Finn had no ill effects from his crash
at the end of first practice,
when he lost control through the penultimate corner and crashed into the wall on the exit of the last corner.
"It's been great," said Hamilton. "Really happy with the way the process and sessions have gone.
"We have some improvements we can make but otherwise all good. This is the first time I've actually felt like P2 has gone well, at least what I can remember. The car feels great here. We look like we're competitive."
The four cars at the front - the two Mercedes and the two Red Bulls - were split by just 0.171secs, while there was a gap back to Massa and the rest of the field.
Alonso was 0.33secs off his team-mate's pace but Ferrari said he had suffered "a little bit of traffic" on his fastest lap.
The Spaniard heads into the race 60 points behind rival Vettel in the championship and appears to be facing another uphill battle trying to prevent the German extending his advantage still further.
"There is room for improvement," said Vettel. "I think it will be very close this weekend with Mercedes. It was close today.
Raikkonen crashes in first practice
Obviously you don't know what other people are doing with fuel loads, etc, but give or take 10kg up or down it is still quite close, and Mercedes always perform very well in qualifying."
BBC F1 technical analyst Gary Anderson said: "The Red Bulls and Mercedes look very close to each other and nipping at their heels are the Ferrari and the Lotus.
"I think we're going to have a really good battle and we won't know who's going to be on pole until the last seconds [of qualifying on Saturday]."
Force India reserve driver James Calado, in his role as BBC F1 analyst, said: "It's going to be really, really close between the Red Bulls and the Mercs. My money's on Hamilton at the moment."
The session was enlivened by a spin from Sauber's Esteban Gutierrez, while Button suffered a big lock-up of his front brakes on his way into Turn One while on his race-simulation run.
Second practice results from Korea in full
Korean Grand Prix, day two
- Saturday 5 October: Final practice at 02:55 BST on BBC Radio 5 live sports extra, online live text commentary from 02:30 BST. Qualifying at 05:55 BST on BBC Radio 5 live sports extra plus live text commentary online from 05:00. Qualifying highlights, 13:15 BST on BBC One.
Korean Grand Prix, day three
- Sunday 6 October: Race coverage at 06:55 BST on BBC Radio 5 live, and live text commentary online from 06:00. Race highlights, 14:00 BST on BBC One.