Mickelson, who has held at least a piece of the lead after each of the first three rounds, struggled to a two-over 72 on Saturday. Love, the 1997 PGA Champion, posted his third consecutive round of two-under 68.
Love's last 54-hole lead in a major was the '97 PGA at Winged Foot, which was also his only major title.
'I enjoy playing with Phil, and Jim, his caddie, is one of my good friends. It'll be a lot of fun,' said Love, who tied for sixth at this year's U.S. Open, but missed the cut in the other two majors. 'We're playing for the PGA tomorrow, so friendship is aside, but it'll be a good day.'
Thomas Bjorn matched several records on Saturday. He fired a seven-under 63 in the third round and vaulted up to third place at five-under-par 205.
The 63 equaled the best round in any major championship and was the ninth time that number was posted in a PGA Championship. Bjorn tied Baltusrol's course record that Jack Nicklaus and Tom Weiskopf shot in the 1980 U.S. Open.
'To post this kind of number is certainly special for me, and I'll certainly remember that for the rest of my life,' said Bjorn. 'It's a long list, but it's a good list to be a part of.'
Tiger Woods, who birdied the 18th hole on Friday to make the cut on the number, carded a four-under 66 on Saturday and moved into a tie for 20th at even-par 210.
Woods bogeyed his first hole, then recorded five birdies the rest of the way. He had several more opportunities, but missed a five-footer for birdie at 12 and 12-footer at 13.
On the par-five 18th, he reached the green in two, but blew his 25-foot eagle try 15 feet past the hole. Woods missed the birdie try and left with a three- putt par.
'It's not about misreading anything that speed. I just hit it way too hard,' said Woods, referring to his three-putt par at 18.
Defending champion Vijay Singh collected 17 pars, then birdied No. 18 for a one-under 69. He is tied for fourth place at minus-four with 1995 winner Steve Elkington (68), Pat Perez (67) and Stuart Appleby (69).
Everyone will be looking up at the American co-leaders.
Love, who began the third round four behind Mickelson, flew out of the gate with a five-foot birdie putt at the first. He made it two in a row with a 15- footer at the second, but things quickly turned for Love.
He fell down the leaderboard with back-to-back bogeys at six and seven, then dropped another shot to par at the 10th to fall to three-under par for the championship.
Luckily for Love, Mickelson was sinking as well.
He made a mess of the second hole en route to a bogey, then three-putted from 35 feet for a bogey at the fifth. Mickelson picked up his second bogey in a row at six when he went from the left rough to a greenside bunker. That bogey dropped him into a tie with Bjorn, who had already been in the clubhouse for several hours.
Love made the first move to distance himself from the rest of the field. He birdied the 11th, then drained a seven-foot birdie putt at the 13th to get within one of Mickelson, who coasted along with pars, including a 12-foot save at the ninth.
Mickelson tallied his first birdie of the round on Saturday at the par-three 12th. He sank the 15-foot putt to move one ahead of Bjorn and Love.
Love rolled in a 25-foot birdie putt at the 15th to join Mickelson atop the leaderboard at minus-six. Love fell out of the tie quickly as he three-putted the 16th green for a bogey.
Baltusrol's par-five closing holes played a key role in the third round. Love ripped a drive down the fairway and knocked his second through the green at the 650-yard 17th. He hit a beautiful pitch that ran three feet past the hole, but he converted the birdie putt to rejoin Mickelson in the lead.
Love parred 18 to get in at minus-six, but with Mickelson's length, one had to expect the lefthander to collect at least one birdie. Mickelson certainly had his chances.
At 17, Mickelson actually laid up short with his second, but knocked a wedge to six feet. He missed that putt, but still had the reachable 18th. Mickelson drove it into the left rough, then hit his second over the green. He received a drop as he was near a tower, then pitched his third five feet short. Mickelson missed that short one and so it is a tie with one round to play.
'To still be in the lead is a big bonus,' admitted Mickelson, whose best finish at the PGA was second in 2001. 'I missed a couple of short putts on 17 and 18, that were tough, but I still could have made them. I fought hard to stay in the lead.'
The final pairing is an interesting twosome. They are Ryder Cup and Presidents Cup partners, but both are more known for their unfortunate lack of success in the majors.
Mickelson and Love have combined for 44 PGA Tour victories, but only two major championships. Each carried the tag, 'Best Player to Never Win a Major,' but each has put themselves in position for major No. 2.
'I've been disappointed,' admitted Love, referring to his poor record in majors. 'You arrogantly think that if you win one, the rest are easy. The second one is just as hard.'
Mickelson decided not to speak about a possible second major.
'I've got a lot to worry about for the next 18 holes, and the last thing I want to do is jump ahead,' said Mickelson.
Two-time U.S. Open winner Retief Goosen (69), 2003 British Open winner Ben Curtis (67), Jason Bohn (68), Greg Owen (70) and Lee Westwood (71) are knotted in eighth at minus-three.
Jerry Kelly, who was alone in second after the second round, shot a four-over 74 and is part of a group tied for 14th place at one-under-par 209.