The Mercedes drivers completed a total of 103 laps in today's three hours of running time, seven laps short of two race distances.
Nico Rosberg conducted a single evaluation run of the team's passive drag reduction device this morning in first practice.
Tyre evaluations and set-up fine-tuning were the order of the day, with both cars in the top four in both sessions.
My only concern after the long runs was graining on the front tyres, which is something we need to work on tonight and watch out for. We have brought also some new bits and pieces to Korea which are working well and leaves me optimistic for a strong result this weekend."
Lewis Hamilton: "I was very happy with how the sessions went today and it's the first time I can remember that second practice has really gone well for me. We haven't changed anything but the day just went smoothly and all the processes with the team worked well. I love this track: it was clean from the start, it provides good racing on Sunday afternoon and it just gets harder and harder through the lap.
You have the first sector which is good for overtaking, then you get more and more corners coming at you and those final two sectors are all about precision. You need a good downforce package, too, and the car is feeling good. We can still make improvements overnight, and I have some things I want to do with the set-up, but it's been a good start for us."
Ross Brawn: "We made a good start to the weekend with both cars today. We ran through the planned programme, evaluated our test items and got good long runs on both types of tyre in the afternoon session to give us plenty of data to pore through this evening. In terms of competitiveness, all the usual Friday caveats apply: we know only what fuel loads we ran, not those of our competitors, so tomorrow will show us where we actually stand. As always, we will work hard overnight to further improve the cars and build on this good start to the weekend."
Mercedes AMG Petronas