McIlroy tees off in shootout at rain-soaked PGA
Rory McIlroy of Northern Ireland catches a golf ball on the practice range during the final round of the 96th PGA Championship at Valhalla Golf Club on August 10, 2014 in Louisville, Kentucky - by Andy Lyons
Three weeks after winning the British Open at Hoylake, the 25-year-old from Northern Ireland was set to race darkness in the last pairing in hopes of avoiding a Monday finish and capturing his fourth major crown.
Birdie was the new par at the 7,458-yard layout thanks to rainstorms that have dumped 2.2 inches on the course over three days, Sunday's deluge flooding water-logged Valhalla to halt play for nearly two hours.
Standing water turned fairways into rivers and swamped greens, backstrokes looking more likely than golf strokes.
But once play resumed, major scoring opportunities were clearly on offer from receptive greens. The course had already yielded a record-low round average of 69.57 shots on Saturday.
Ernie Els birdied four holes in a row from the third to sixth holes while fellow South African and major winner Charl Schwartzel ran off six birdies in 11 holes to match Els on nine-under.
McIlroy launched his charge at 13-under 200 with Austrian playing partner Bernd Wiesberger one stroke adrift.
McIlroy will try to complete a hat-trick of victories after winning the British Open three weeks ago and a World Golf Championships event last week, what would be the longest win streak since Tiger Woods completed a run of five in a row in 2008.
Five-time major champion Phil Mickelson opened with a 30-foot birdie putt to a roar from the crowd, reaching 11-under.
Sweden's Henrik Stenson and Finland's Mikko Ilonen opened with birdies to reach 10-under.
They along with McIlroy and Wiesberger could make this the first year in which Europeans have won three of the four majors, a good omen for the trophy holders entering next month's Ryder Cup matches with the United States at Gleneagles, Scotland.
If he wins, McIlroy would become the fourth-youngest man to win his fourth major, trailing only Young Tom Morris, Tiger Woods and Jack Nicklaus, and have taken four of the past 15 majors contested.
McIlroy, whose other major wins came at the 2012 PGA Championship and 2011 US Open, would be the first man to win two majors in a row or in the same year since Ireland's Padraig Harrington took the British Open and PGA in 2008.
Only Woods and Mickelson have won a major title after claiming a victory the week before.
McIlroy would join Harrington, Woods, Walter Hagen and Nick Price as the only men to win the British Open and PGA in the same season.