McIlroy 1 clear of Day at Valhalla
The 2012 PGA Champion finished 36 holes at 9-under-par 133. McIlroy has won his last two starts at the Open Championship and the WGC-Bridgestone Invitational.
A pair of accomplished players moved within one of McIlroy's lead. Jason Day, who won the WGC-Accenture Match Play Championship in February, fired a 6-under 65, the low round of the day, to jump into a share of second at minus-8. Day was joined there by former U.S. Open champion Jim Furyk, who shot 68 in round three.
Rickie Fowler, who has finished inside the top-5 at each of the first three majors, fired a 5-under 66 to jump into a tie for fourth at 7-under 135. He stands alongside first-round co-leader Ryan Palmer (70).
Phil Mickelson carded a 4-under 67 to end 36 holes at minus-6.
Steve Stricker (68), Henrik Stenson (71), Louis Oosthuizen (67) and first- round co-leader Lee Westwood (72) are among eight players tied for eighth at 5-under 137.
Four-time PGA Champion Tiger Woods birdied two of his last four holes, but that only got him back to 3-over for his round. He posted his second straight 74 to end at 6-over 148. He missed the cut by five strokes. That was just the fourth time as a professional that Woods missed the cut in a major.