Scott, the joint-overnight leader alongside American Jim Furyk, fired a two-under 68 to grab a one-shot clubhouse lead over a hard-charging Rose, who staged a spectacular back nine rally to return a four-under 66.
The morning wave was greeted at stately Oak Hill Country Club by dark skies and pounding rain which toughened the playing conditions but left the challenging East Course more receptive to low scoring.
Scott teed off on the back nine and picked up a birdie on his opening hole and another at the par-5 13th after rolling in a slippery 15-footer. The Australian notched his third birdie at 16 but then missed a five-foot par putt at 17 for a bogey that moved him to two-under at the turn.
As the rain continued, Scott slogged his way to the finish with two birdies offset by two bogeys but good enough to leave him alone with the clubhouse lead.
“It was raining pretty hard on and off and the course was playing tough,” Scott told reporters. “So it was nice to get off to a good start while the tough conditions were out there and I managed to hang on. The course is obviously softening up and I’m sure some guys will take advantage of that but so far so good for two days.”
Rose, playing in the same group with Scott, gave chase after the turn with a flawless display that included six birdies over his final nine holes.
Robert Garrigus and Furyk were among the afternoon starters sitting two off the pace on five-under.
American Webb Simpson made a dramatic charge in the drizzle by rocketing up the leaderboard with a six-under-par 64 that equalled the course record.
On seven-under with three to play, Simpson had looked poised to pen a bit of golf history and become the first player to card a 62 at one of golf’s four majors. But his flirtation with the record books ended at the seventh hole where he picked up his only bogey of day.
Germany’s Martin Kaymer, a former world No 1 and the 2010 PGA champion, joined Simpson on four-under after returning a second consecutive 68.
Defending champion Rory McIlroy (71), eager to turn his game around after a poor season by his standards, was heading for an early exit until a late turnaround down the stretch. The Northern Irishman’s play was as gloomy as the early weather until he roared back with four birdies over his final seven holes to move comfortably above the projected cutline.
Tiger Woods, stuck in a five-year major-victory drought, was among the late starters. The heavy favourite coming into this week after romping to a seven-stroke victory at the WGC-Bridgestone Invitational on Sunday, signed off with a double-bogey on his final hole for an unsatisfying one-over 71 in his opening round.