Knost alone in front after 66
While Knost held a two-shot lead over Carl Petterson, Luke Donald was three under after seven holes as he looks to retain his world number one slot.
The Englishman needs to finish in the top eight at Harbour Town to remain ahead of Ryder Cup team-mate Rory McIlroy in the standings. He struggled to a four-over-par 75 on Thursday but improved early yesterday with three birdies in a bogey-free first seven holes.
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Hide AdKnost started the day in a three-way tie for the lead with Chad Campbell and Vaughn Taylor. The American made an ideal start with birdies at the first two holes before finishing a bogey-free round with five birdies overall.