Having seen his run of Friday domination ended by Lewis Hamilton on the opening day of the Korean Grand Prix, Sebastian Vettel was keen to lay down a marker ahead of qualifying for this weekend's event.
The world champion ended the hour-long Saturday morning session fastest by just over a tenth of a second, but it was Red Bull team-mate Mark Webber in second spot and not the Mercedes, although the Australian will incur a ten-place grid penalty in qualifying following his 'hitch-hiking' episode last time out in Singapore.
More importantly than the gap between the two Red Bulls was the four-tenths deficit to the best of the Mercedes, which had appeared to have the measure of the Milton Keynes team on day one. With Hamilton suffering both a brake problem early on and traffic while trying to go for a flying lap at the end, it was Nico Rosberg who wound up as the closest challenger to the pacesetters, lapping in 1min 38.3secs to Vettel's 1min 37.8.
Hamilton took fourth place, 0.014secs slower than his team-mate, whom he had inadvertently held up during the German's first flying run, while Ferrari and Lotus filled the next four spots, lining up symmetrically with Fernando Alonso heading Romain Grosjean, Felipe Massa and Kimi Raikkonen, while Nico Hulkenberg gave Sauber a fillip in ninth, ahead of Jenson Button's McLaren.
The second Sauber of Esteban Gutierrez was next up, underlining the young Mexican's season-long improvement, while Pastor Maldonado took twelfth for Williams after getting exclusive use of the team's latest front wing. Team-mate Valtteri Bottas was 18th fastest, just ahead of the expected tailenders, where Charles Pic enjoyed a three-tenth advantage over Caterham team-mate Giedo van der Garde and Marussia's Jules Bianchi.