-7 P Harrington (Ire) -6 T Clark (SA) -5 S Verplank (US) -4 P Marksaeng (Tha), S Stricker (US), J Kelly (US) Selected others: -2 T Woods (US), O Wilson (Eng) -1 P Mickelson (US) Even N Dougherty (Eng), L Westwood (Eng)
Harrington has been struggling with his remodelled swing
Overnight leader Padraig Harrington shot a 69 to maintain his clubhouse lead after two rounds of the World Golf Championship event in Ohio.
The Irish three-time major winner picked up where he left off on Thursday to sit seven under, a shot clear of Tim Clark at the Firestone Country Club.
Scott Verplank matched playing partner Harrington with a round of 69 that leaves him third on five under.
England's Oliver Wilson is two under, alongside world number one Tiger Woods.
Harrington had produced his best golf of the year with an opening-round 64 to lead by two from South Africa's Clark, American Verplank and Thailand's Prayad Marksaeng, who now sits three shots off the lead on four under.
The 37-year-old Irishman began his second round well, moving to seven under par after five holes before bogeying the par-four 8th - the only blemish on his round.
Harrington, preparing for the defence of his USPGA title at Hazeltine in Minnesota next week, bounced back at the 13th courtesy of a birdie from 30 feet.
"I was a bit cautious today. I was more worried about my misses rather than having that sort of confident attitude and going after a few more pins
"Shooting that score yesterday gave me a great chance for the tournament," said Harrington, who has been struggling with his remodelled swing.
"Anything under 70 from here on in is going forward so I'm quite pleased to have shot 69."
The Dubliner admitted his second round had been less aggressive than his first but added: "I normally play the first rounds cautiously.
"I was a bit cautious today. I was more worried about my misses rather than having that sort of confident attitude and going after a few more pins.
"You know that if you miss the green on the short side you're going to have difficulty getting up and down. But you've got to make some birdies.
"My short game has been good enough, so I probably should be a little bit more aggressive over the weekend."
Clark, starting his round at the 9th, sank birdies at 11, 15 and 18 to make the turn at seven under, but bogeyed the 3rd to slip back to second.
Marksaeng got his second round under way with birdies at the 2nd, 5th and 7th to join Harrington at seven under.
But he endured a poor back nine, bogeying the 11th, 14th and 18th and goes into the weekend in a tie for fourth alongside Americans Steve Stricker and Jerry Kelly.
Kelly fired a 71 on Thursday but responded with the joint-best score on Friday, a 65, which included five birdies and 13 pars.
The other player to shoot a five-under round of 65 was Spain's Alvaro Quiros.
Quiros made his big move into contention with two birdies on the front nine and three more on the back to move from two over to three under.
Woods, a six-time winner at Firestone, recorded a 70 to stay two under, while playing partner Sergio Garcia carded a 72 to fall back to level par.
Four birdies in his first 12 holes moved Wilson to five under but three bogeys on his back nine left the Ryder Cup player with a 69.
Open champion Stewart Cink finished at two under alongside Woods and Wilson but England's Ian Poulter sits one over after slumping to a 74.
Compatriots Nick Dougherty and Lee Westwood both shot 71 to return to level par, while Northern Ireland's Darren Clarke, the 2003 champion at Firestone, shot a 70 and is one over.