Rory McIlroy
McIlory leads nine under par.Getty

Rory McIlroy is on course to secure back to back majors after taking a half way lead at the US PGA Championship at Valhalla.

2012 champion McIlroy went into the weekend nine under par after scoring four under par 67 on Friday to stay one shot clear of second placed Jason Day (65) and Jim Furyk (68).

"I didn't get off to the best of starts but I righted the ship well, caught fire a little bit round the turn and am really happy with four under par," McIlroy was quoted as saying by BBC Sport.

"I'm feeling good about my game, I'm confident and hitting the ball well for the most part. I'm in control of my game and my emotions and I will need to do that over the weekend as well."

Rickie Fowler (66), Ryan Palmer (70) and Mikko Ilonen (68) sit joint third on seven under, followed by 2013 Open champion Phil Mickelson on 67.

Elsewhere, Tiger Woods missed the halfway cut after finishing six over par.

The 38-year-old was a major doubt for the final major of the year after withdrawing from the WGC-Invitational with a back problem just weeks after returning to golf following four months out which saw him miss The Masters and The US Open, but recovered in time for The Open Championship last month.

The 14-time major winner scored a second round 74 and admitted he struggled after last week's injury.

"It was sore. It went out on me on the range," he said.

"When I was warming up I hit a four iron and felt the same thing I had last week. It's a matter of getting through it."