Lewis Hamilton completed a clean sweep of fastest times in Korean Grand Prix Friday practice as Mercedes again took the fight to Red Bull.
In another strong hint that the champion team will face tougher opposition this weekend than of late, Hamilton not only set the outright pace with a 1m38.673s, but was also very positive about tyre life over later long runs.
Sebastian Vettel was 0.108 seconds down on Hamilton in second place.
The runaway world championship leader, chasing a fourth consecutive Formula 1 race win this weekend, waited much longer than anyone else to move to super-soft tyres, jumping to second when he did so.
That denied Mercedes a one-two, with Vettel edging Nico Rosberg by 0.016s.
Mark Webber had been Hamilton's main rival early on, as the pair swapped first place several times. The second Red Bull ended up back in fourth, but was within 0.063s of its sister car.
Felipe Massa was Ferrari's pacesetter in fifth, two places ahead of team-mate Fernando Alonso. Romain Grosjean's Lotus was in between them.
The second Lotus of Kimi Raikkonen got straight back out after its morning crash and went eighth.
Despite a cockpit niggle, Jenson Button was ninth for McLaren, ahead of very-closely matched Force India duo Adrian Sutil and Paul di Resta, who were just 0.001s apart.
No one repeated Raikkonen's trip into the wall, but both Caterham's Charles Pic and Sauber's Esteban Gutierrez had spins during the afternoon while Alonso visited the grass on his final lap.