Circuit ace Kostka posted the third best time on SS3 Košíky, the longest test of the event at 22.22 kilometres. That leaves him just 0.4 seconds behind third-placed Kajetan Kajetanowicz overall.
Jan Kopecký took his third fastest time from as many stages, with Alexey Lukyanuk close to him once more. The pair are now separated by 3.5s.
Bryan Bouffier has climbed to fifth ahead of Pavel Valoušek. Jaromír Tarabus is up to seventh in front of Jan Cerný, Ralfs Sirmacis and Fabian Kreim.
Hermen Kobus meanwhile has crashed out of eighth place overall in his ŠKODA.
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The crews now return to service in Otrokovice before the Hošťálková stage at 13h02 local time.