Los Angeles (AFP) - Jimmy Walker, gearing up for his PGA Championship title defence, fired a five-under 65 on Friday to take a two-shot lead at the halfway stage of the World Golf Championships Bridgestone in Akron, Ohio.

He shook off lengthy weather delays as thunderstorms disrupted action at Firestone Country Club.

He carded six birdies with just one bogey en route to a 36-hole total of seven-under 133.

He edged ahead of overnight leader Thomas Pieters of Belgium, who had two birdies and two bogeys in a 70 for 135.

Rory McIlroy, Zach Johnson and Hideki Matsuyama were a further stroke back.

McIlroy, playing his first tournament since sacking longtime caddie J.P. Fitzgerald as he tries to turn around a so-far disappointing season, chipped in for birdie at the 16th en route to a one-under 69 while Japan's Matsuyama and America's Johnson both shot 67.

Australian Jason Day turned in a four-under 66 to join a group of six players on 137 that also included British Open champion Jordan Spieth, who signed for a 70.

Day had charged up the leaderboard with four birdies in his first five holes and five birdies on the front nine. But he appeared to tweak his troublesome back, and had two bogeys and a birdie coming in.