Lewis Hamilton put in a late charge to pip Fernando Alonso to the P1 slot in Friday's first practice for the Korean GP.
Although it was the Championship leader who held onto the P1 slot for most of the session, always seeming to improve his time when his rivals closed the gap, it was Hamilton who ended FP1 quickest of all.
The McLaren driver posted a 1:39.148 to edge Alonso by 0.302s. Mark Webber was third.
It was a relatively cool start to the morning with Valtteri Bottas posting the first lap time, a 1:45.225, which, despite going quicker, was easily beaten by Jenson Button by 2.350s.
With the duo doing their part to clean up the dusty Yeongam circuit, others ventured out as the day's action kicked off in earnest.
Fernando Alonso was the next to lead the way with a 1:40.895 while others struggled to stay on the track which was slippery because of the dust. Test drivers Giedo van der Garde and Jules Bianchi both had moments as did Felipe Massa and Sergio Perez.
Halfway through the session the top three were Alonso, Pastor Maldonado and Michael Schumacher.
Alonso's time stood strong while his rivals slowly but surely closed the gap as Nico Rosberg was the first to get within a second of it.
Mark Webber posed the next challenge only for Alonso to extend the gap as he went four-tenths quicker than previously. Lewis Hamilton closed the gap with 30 minutes remaining as he moved up to second, 0.025s off the pace.
With drivers pushing hard in the final 15 minutes, Rosberg ran off the track as did Perez, van der Garde kicked up dirt as he rode the kerbs hard and Alonso locked up, had a serious slide and complained of "a lot" of vibrations.
The weekend's first practice ended with Hamilton putting in a late charge to take the P1 slot with a 1:39.280, relegating Alonso to second by 0.302s.
01 Lewis Hamilton McLaren 1:39.148 23 laps
02 Fernando Alonso Ferrari 1:39.450 0.302 21 laps
03 Mark Webber Red Bull 1:39.575 0.427 21 laps
04 Felipe Massa Ferrari 1:39.854 0.706 23 laps
05 Sebastian Vettel Red Bull 1:40.088 0.940 21 laps
06 Michael Schumacher Mercedes 1:40.221 1.073 21 laps
07 Nico Rosberg Mercedes 1:40.396 1.248 24 laps
08 Romain Grosjean Lotus 1:40.422 1.274 22 laps
09 Paul di Resta Force India 1:40.440 1.292 19 laps
10 Jenson Button McLaren 1:40.480 1.332 21 laps
11 Kimi Raikkonen Lotus 1:40.929 1.781 14 laps
12 Pastor Maldonado Williams 1:41.048 1.900 25 laps
13 Jules Bianchi Force India 1:41.140 1.992 21 laps
14 Kamui Kobayashi Sauber 1:41.220 2.072 19 laps
15 Sergio Perez Sauber 1:41.514 2.366 20 laps
16 Daniel Ricciardo Toro Rosso 1:41.596 2.448 23 laps
17 Jean-Eric Vergne Toro Rosso 1:42.021 2.873 25 laps
18 Valtteri Bottas Williams 1:42.027 2.879 23 laps
19 Heikki Kovalainen Caterham 1:42.104 2.956 24 laps
20 Timo Glock Marussia 1:42.175 3.027 13 laps
21 Charles Pic Marussia 1:42.706 3.558 22 laps
22 Giedo van der Garde Caterham 1:42.820 3.672 19 laps
23 Pedro de la Rosa HRT 1:44.517 5.369 23 laps
24 Dani Clos HRT 1:45.735 6.587 22 laps