The unheralded Micheel was on fire with five birdies in a nine-hole stretch but had trouble negotiating the demanding finishing stretch at punishing Oak Hill as he was forced to settle for a 1-under-par 69.
The equally winless Campbell birdied the last hole for a 65 - the best round of the tournament - to join Micheel at 4-under 206 with 18 holes to play.
World No. 2 Ernie Els of South Africa managed a 70 and was among a group five shots off the pace. Tiger Woods, however, all but conceded defeat after shooting a 3-over 73 to plunge 13 shots off the pace.