Jimmy Walker fired a 65 to lead the way on a weather-affected second day of the WGC-Bridgestone Invitational, but had Thomas Pieters and Rory McIlroy breathing down his neck at Firestone Country Club.

After a 45-minute delay at the start of the day due to the threat of lightning, two further suspensions saw four hours and five minutes lost with a spell of torrential rain in the early afternoon.

When the hooter sounded for the second time to spark a three hour and 19 minute delay, Walker and Pieters were sharing the lead at five under, but the American picked up two birdies on the back nine to open up a two-shot advantage.

McIlroy was then in the group at four-under after a 69, alongside World No 3 Hideki Matsuyama and 2015 Open champion Zach Johnson.

Russell Knox was a further shot back alongside Alex Noren, with Tommy Fleetwood then at two-under.

The nearest South African challenge came from Charl Schwartzel, who sits six shots off the lead after his 69. An early bogey was offset by birdies at 13, 15 and 16 to get him back under par for the tournament.

Branden Grace is level after 36 holes, after a second round 69 which included four birdies – crucially, two of those came in succession in the final three holes.

Louis Oosthuizen is +6 at the halfway mark with Shaun Norris, dropping 35 spots on the leaderboard after his 77 on Friday.

Walker – who will defend his US PGA Championship title next week – has struggled this season after suffering from Lyme disease, but showed all the qualities on Friday that made him a Major champion as he brushed off all the distractions to fire the lowest round of the day.

‘It hasn’t been a lot of fun this year,’ he said. ‘I had a few okay finishes, a lot of 30s, high 20s, just stuff that’s not moving me around.

‘It’s nice to see some putts go in and make some solid swings and keep rounds going, make some par putts, just the stuff I haven’t been doing.’

The 38-year-old made a fast start with birdies on the second, third and fifth, and while he dropped a shot on the seventh, he regained it on the next. His back nine was then a tale of the par threes as he put his tee shot to the 12th to two feet to take the lead, and his first to the 15th to eight feet.

The latter birdie moved him back ahead, after Pieters had caught him at the top of the leaderboard with the help of a remarkable shot on the tenth.

The Ryder Cup star entered the day with a one-shot lead, and while he started with two poor shots to bogey the first, he took advantage of the par five next to sit level par for the day at the turn. He found sand off the tee on the first hole after the turn, but hit a stunning bunker shot round the trees and to five feet to keep the pressure on Walker.

Pieters was not so impressive from the sand at the 15th, however, and after he failed to get up and down, Walker led by two.

‘I played decent,’ said Pieters. ‘Again, I think I hit plenty of greens, I think a bit less than yesterday.

‘The driver was still a little bit off. Ball-striking’s been good, the same today. I missed a couple of short ones on 17 and 18, but I’m rolling the putter nicely, so I’m looking forward to this weekend.’

McIlroy was another player to capitalise on the gettable second, but he failed to get on in regulation and dropped a shot on the ninth. He followed that with six pars, and after finding the rough with his first three shots on the par five 16th, the four-time Major winner chipped in for a birdie.

Johnson had five birdies and two bogeys in a 67, while Matsuyama posted the same score with three birdies in his last seven holes.

Scotsman Knox had been seven-under after starting with three birdies, but a hat-trick of bogeys from the 11th saw him tumble down the leaderboard.

Swede Noren was over par at the turn, but birdies on the 11th, 13th, 16th and 17th sent him in the other direction.

Australian Jason Day turned in 30, but looked to be struggling with his back in a 66 that got him to three-under, alongside Champion Golfer of the Year Jordan Spieth, fellow American Charley Hoffman and Canada’s Adam Hadwin.

Hudson Swafford made the 25th hole-in-one of the season, and the first here since 2011 as he aced the 15th from 214 yards.