Rory McIlroy recovered from a slow start to claim the clubhouse lead in pursuit of his second major title in succession on a weather-affected second day of the 96th US PGA Championship.
McIlroy briefly dropped three shots off the pace when he bogeyed the par-four 12th, his third hole of the day, but bounced back to card a second round of 67 at Valhalla.
The 25-year-old's 12th successive sub-par score gave him a nine-under-par halfway total of 133, two shots ahead of American Ryan Palmer. Ireland's Padraig Harrington was the last man to win back-to-back majors in the 2008 Open and US PGA.
"The golf course was pretty wet so the ball is not running anywhere and whenever it's like that and no rain is coming down it is very playable and you could make a score," McIlroy said.
"I didn't get off to the best of starts, but I righted the ship well and caught fire a little bit round the turn and am really happy with four under par.
"I'm feeling good about my game, I'm confident and hitting the ball well for the most part - not quite as well as I did yesterday, but still in control of my game and my emotions and I will need to do that over the weekend as well."
McIlroy had not looked in top form in the early stages of his round, pushing his opening drive into the crowd on the par-five 10th and failing to convert a birdie putt from 10 feet, although a par was at least two shots better than he managed there on Thursday after hooking his second shot out of bounds.
Another birdie chance went begging on the 11th and the world number one then bogeyed the 12th, pulling his drive into the gallery and failing to get up and down from a greenside bunker.
McIlroy, who followed his Open victory with a first World Golf Championship title in the Bridgestone Invitational on Sunday, got back on track with birdies at the 13th and 15th and missed from six feet for another on the 16th after a towering four-iron approach from 224 yards.
But he made amends by holing from 30 feet for an eagle on the 18th and could have had another on the seventh, missing from eight feet after a stunning three-wood approach. As if to emphasise a changing of the guard, his closing birdie on the ninth came just as former world number one Tiger Woods began his second round on three over par.
Play had got under way in torrential rain at 7:30am but was suspended just 20 minutes later with the first group of joint overnight leader Palmer, Ireland's Shane Lowry and Bob Sowards still on the first green.
Two members of the green-keeping staff at Valhalla had been attempting to clear water from the putting surface with squeegees, but that proved an impossible task as the rain intensified.
The rain eased enough for tournament officials to announce play would restart after a delay of 45 minutes, although preferred lies could not be used as they were surprisingly not in force when the round got under way.
"It was tough when we teed off," Palmer said after adding a 70 to his opening 65. "I wasn't quite sure if we should have teed off to be honest with you, you could barely see the fairway. It was tough when we got to the first green, we couldn't get a clean lie with the standing water, so they couldn't even squeegee it clear enough.
"We were questioning in the fairway why we had to hit into a puddle because the first green was already under water. They said, 'That's just golf'. We're like, 'Tell the guy indoors that's making the decision to come check it out, because it's wet'."