South Africa's Louis Oosthuizen and American Brooks Kopeka edged ahead of clubhouse leader Phil Mickelson late in Friday's second round of the PGA Championship at windy Kiawah Island.
Oosthuizen, the 2010 British Open champion, birdied three of the first six holes and added another at the par-5 11th to stand on 6-under, going 20 consecutive holes without a bogey on the treacherous Ocean Course.
Four-time major winner Koepka, playing with pain after right knee surgery in March, eagled the par-5 11th and birdied the par-4 12th to match Oosthuizen for the top spot.
But both still had the formidable 14th through 18th holes to play into the gusting coastal wind that has played havoc with some of the sport's top stars.
Mickelson, chasing a historic sixth major title at age 50, birdied five of his last eight holes to seize the clubhouse lead on 5-under 139 for 36 holes.
The American left-hander rolled in a birdie putt from just inside 23 feet to cap a three-under par 69 second round.
"I'm heading into the weekend with an opportunity and I'm really excited about it," Mickelson said. "If you told me (I was leading) Sunday night, I'd really enjoy it. Right now, there's a lot to do."
A victory would make Mickelson the oldest champion in major golf history, erasing the mark set by American Julius Boros when he won the 1968 PGA Championship at age 48.
With afternoon starters facing the day's highest wind gusts, Mickelson looked likely to become the first 50-and-over player in the top five of a major after 36 holes since American Fred Couples at the 2013 Masters.
Mickelson would be the first 50-and-older player entering the weekend in the top five at the PGA since Hale Irwin in 1999.
Since the PGA Championship adopted a stroke-play format in 1958, the only 50-and-over player to lead or share the lead after 36 holes was Sam Snead in 1966 at age 54.
Momentum swung within moments in the early afternoon as Mickelson sank his birdie putt at 18, a Covid-19 limited crowd of about 10,000 roaring in delight.
"It's really fun to make a putt on the last hole and have a round like that," Mickelson. "The support here has been really special."
South African Branden Grace followed by plunking his tee shot into the water at the par-3 17th on the way to a double bogey, handing Mickelson the lead.
A bogey at 18 left Grace, who holed out from 70 feet from sand to birdie the par-4 10th, with a round of 71 and second in the clubhouse on 141.
"Just trying to stay alive out there," Grace said. "It was such a grind."
Mickelson, who began off the 10th tee, put his approach inches from the cup to birdie the par-5 11th but found water at the par-4 13th and saved bogey with an approach eight inches from the hole.
But Mickelson became tentative with the putter into severe headwinds and made bogeys at 17 and 18 to fall back.
"When I made a few iffy strokes on 17 and 18 and made bogeys, I still had to be patient," Mickelson said.
Mickelson fired a 31 on the front nine, holing birdie putts from inside five feet at the second and fourth, dropping a 15-footer at the par-3 fifth and landing his approach inches from the cup to birdie the par-5 seventh before his dramatic finish.
"I've driven it well, but I think the thing I've done the best is my brother (and caddie) Tim and I have done a really good job of judging the wind and picking clubs with the right flight to get the right distance," said Mickelson. "We've hit a lot of iron shots pin high."
Grace, the Puerto Rico Open winner in February, fired a major record-low round of 62 at the 2017 British Open at Royal Birkdale. His best major finish was third at the 2015 PGA.
South African Christiaan Bezuidenhout sank a 31-foot eagle putt at the par-5 seventh and shot 70 to share second in the clubhouse with Grace on 141.
- DJ likely to miss cut -
Several players still had to fight howling winds to make the cut to the low 70 and ties.
Dustin Johnson fired a 74 to stand on 150 and appeared destined to become the first world number one to miss consecutive major cuts since Greg Norman in 1997 at the Masters and US Open.
Three-time major winner Jordan Spieth, chasing a victory to complete a career Grand Slam, shot 75 to stand on 148, and was likely to reach the weekend.