Phil Mickelson, the reigning Masters champion, flew up the leaderboard with a 5-under 31 on the front side. He shot a 5-under 67 and is tied for third place with Stephen Ames (69), Darren Clarke (72), Ernie Els (72) and Chris Riley (69) at 8-under-par 208.
Tiger Woods barely extended his PGA Tour record for consecutive cuts made on Friday. He birdied two of his final three in the second round to push the number to 129.
Woods carded a 69 on Saturday, but never got anything going on the back nine. He is tied for 25th place at minus-3 and is in danger of running his majorless streak to 10.
'If I could get three or four more on the back nine, I would be right back where I needed to be,' said Woods, who won this title in 1999 and 2000. 'We need some help from the leaders.'
Singh and Leonard showed few signs in the third round that they will be coming back to pack. The duo shared the 36-hole lead and Leonard flinched first with a bogey at No. 1.
Leonard, the 1997 British Open champion, took first place with his play through the opening 10 holes. He collected a pair of short birdie putts at five and six, then knocked his approach to 4 feet at the 10th to reach 11 under par.
Leonard hit a pitching-wedge to 6 feet at the short, par-3 12th to set up birdie. He was now two clear of Singh, but the No. 3 golfer in the world was waiting to move up the leaderboard.
Singh drained an 18-foot birdie putt at the 13th to close the gap to a stroke. Leonard drove into the right rough at the 15th and was forced to lay up short of the green. He hit his third to 15 feet, but missed the putt, bogeying the hole and falling into a tie for the lead with Singh.
Both players found the fairway at the par-5 16th and Leonard's second came up short of the putting surface. Singh went after the green in two with a 5-wood, but his shot came to rest against the lip of a bunker in front of the green. Leonard pitched to 4 feet, while Singh could only manage to blast his 20 feet short of the stick. Singh ran home his long birdie putt and Leonard rolled his in on top of Singh and the duo remained knotted at minus-12.
The pair made routine pars at the par-3 17th, but Whistling Straits' difficult closing hole awaited. Each player drove in the short grass, but Leonard's second fell into a greenside bunker. Singh's approach came up 35 feet short of the hole.
Leonard barely advanced his ball past the fringe, leaving himself with about 40 feet. He narrowly missed that putt and tapped in for bogey. Singh's birdie effort came up 6 feet short, but he calmly holed the par-saver to take first by himself.
'I'm quite happy with the way I'm playing,' said Singh, who also won the 2000 Masters. 'I like playing with Justin and I was just having fun out there. I made some really good putts.'
Singh will have plenty going for him come Sunday afternoon. He is 10-7 all-time on the PGA Tour with a piece of the 54-hole lead, including seven wins in his last seven opportunities. The last time Singh did not win when he held at least a share of the third-round lead was the three years ago at the event in Hilton Head.
Also going for Singh is the history of players with the 54-hole lead at the PGA Championship. In the last seven years, a golfer who has part of the third-round lead has won six times. The only player who squandered the advantage was Leonard in 2002, when Rich Beem took the title.
'I think there's five other guys that are four back now,' said Singh. 'They have to play one shot better to catch me now. They have one more shot to catch up, and that meant a lot to me.'
Singh has four victories this season and if he hoists the Wanamaker Trophy Sunday afternoon, it will be the second time. It will also be his third major and Singh, despite not taking it as seriously as some, is excited about the prospect.
'Majors are important to me. Probably not as important as the other guys, but I strive to win majors,' said Singh. 'I'm going to go out and do my hardest. I'm going to try to beat everybody in the field if I can.'
Leonard is the closest in the rearview mirror, and despite not playing his best golf in 2004, he thinks he learned some valuable lessons from his final-round meltdown at Hazeltine.
'I've gained a lot of perspective in the last couple of years, since Hazeltine, and what I do tomorrow is not going to define the player that I am,' said Leonard, who finished as the runner-up to Davis Love III in 1997. 'I think that if I can keep that in my thoughts, then I'll have a better chance of doing the things that I've done and not worrying about trying to win a golf tournament.'
Els and Briny Baird held a piece of the lead at various points in Saturday's third round, but each struggled down the stretch. Baird triple-bogeyed the par-3 17th and Els bogeyed 18 after a poor club selection off the tee and a chip on the green.
Baird struggled to a 3-over 75 and is tied for 11th at 5-under-par 211.
Chris DiMarco will try to accumulate some much-needed Ryder Cup points on Sunday. In the third round, DiMarco carded a 1-under 71 and is alone in eighth place at 7-under-par 209.