Oct.22 (GMM) The opening sector of the Yeongam circuit in Korea looks set to be a headache for Red Bull this weekend.

Although grounded for most of the opening session on Friday with mechanics working at the rear of his car, McLaren's Lewis Hamilton still managed to top the timing sheet.

The fastest Red Bull was Sebastian Vettel's, half a second behind and fourth, with the now extremely-efficient F-duct on Renault propelling Robert Kubica to the second best time.

The problem for Red Bull in the first sector is the ultra-long straight, with Ferrari also proving faster there.

"Sector one doesn't look like our home ground," agreed Vettel, "but I think sector two and sector three should give us a possibility to catch up."

His teammate Mark Webber, recalling August's Belgian grand prix where Hamilton won, added: "It was the same at Spa. We knew that we were going to be quite vulnerable up the hill there."

Indeed, in Korea, the MP4-25 reached 316kph on the long straight, nearly 10kph faster than Red Bull's RB6.

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