Hamiltonâ€™s time of 1m50.828s was just 0.104s quicker than team-mate Jenson Button as the McLaren duo traded fastest laps, with newly-crowned double world champion Sebastian Vettel a distant third, 1.8s slower.
A saturated track for the first session meant little action before Hamilton made an exploratory venture out of the garage – and he was soon followed by others on full wets to get some wet weather running.
Yeongam enjoys a micro-climate and despite forecasts showing a dry race on Sunday, rain could still be a factor.
Hamilton took some encouragement out of his form in Fridayâ€™s practice, but was still realistic as he got the better of Button, who had outperformed him in practice last weekend at Suzuka, before going on to take the race win.
“Todayâ€™s times donâ€™t really give us a relevant read on pace and set-up, particularly as I hear that the weather will be better for the next two days,â€ Hamilton said.
â€œStill, if it rains on Sunday then I think weâ€™re in a good position â€“ which is a positive.
â€œIt would be great to get the pole here tomorrow: itâ€™s difficult to read too much into todayâ€™s times â€“ the lap on slicks at the end was impossible, but I think we are quick, the car is good and Jenson showed at the last race that we can be very competitive. So I have no doubt that weâ€™ll be able to challenge the Red Bulls in qualifying.â€
Both McLaren drivers were so confident with the MP4-26 on the intermediate tyres that even Vettel was unable to compete with their pace, while Ferrariâ€™s Fernando Alonsoâ€“who won in Korea last yearâ€“was two seconds down in fourth with a 1m52.774s.
Toro Rossoâ€™s Jaime Alguersuari was sixth quickest behind Red Bullâ€™s Mark Webber despite being hit by Nico Rosberg as the Mercedes driver negotiated the notoriously difficult pit exit.
Stewards investigated the incident, but took no further action against Rosberg, citing the Germanâ€™s good record in Formula 1, and data which indicated that Rosberg was on full braking from the time where he apparently noticed Alguersuari.
Rosberg was fined â‚¬10,000 (Â£8,729), â‚¬5,000 (Â£4,364) of it suspended, for failing to report to the stewards in a timely fashion.
Ferrariâ€™s Felipe Massa was seventh with a 1m53.707, two tenths ahead of Rosberg, Toro Rossoâ€™s SÃ©bastien Buemi, and Force Indiaâ€™s Paul di Resta who were separated by less than a tenth of a second.
Michael Schumacher, who produced a late effort to top the timesheets in the first session, was a disappointing 14th quickest, a full four seconds off Hamiltonâ€™s time.
With none of the teams able to do any high-fuel running, Saturdayâ€™s remaining practice session will be crucial to learn more about how the track and the new Pirelli tyres will perform at the Korean International Circuit.
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