Hamilton’s time of 1 minute, 39.630 seconds around the Yeongam circuit was just three-hundredths of a second faster than runaway Formula One championship leader Vettel, with Webber also within two-tenths of a second of Hamilton.
Hamilton’s Mercedes teammate Nico Rosberg was fourth fastest, followed by the McLaren of Jenson Button and Ferrari’s Fernando Alonso.
Hamilton’s ability to hang onto the top time raised hopes of a more competitive race on Sunday, after Vettel had driven out of sight of his rivals to comfortable wins in the past three races.
Vettel has won the past two Korean GP races and leads the 2013 championship from Alonso by 60 points with six races remaining, putting him in a powerful position to notch a fourth-straight title.
Many drivers had excursions off the unrubbered and dusty track, with Lotus’ Kimi Raikkonen having a heavy side impact with a tire wall at the very end of the session after failing to negotiate the final turn approaching the pit straight.
The collision would do little for the state of Raikkonen’s sore back, with the Finn saying on Thursday that his availability for the weekend would hinge on how he feels after Friday’s two practice sessions.
The opening practice also marked the first outing for Lotus’ longer wheelbase car, but the team made no immediate leap forward with Raikkonen eighth fastest, one place behind his teammate Romain Grosjean.
McLaren’s Sergio Perez was ninth on the timesheets, ahead of Ferrari’s Felipe Massa.
A second 90-minute session was scheduled for later Friday, with mild and sunny conditions prevailing in south-west South Korea, though Sunday’s race could affected by a tropical storm forecast off the peninsula’s southern coast.Formula 1, Kimi Raikkonen, Korean GP, Lewis Hamilton, Mark Webber, Sebastian Vettel