Mickelson, winner of the Professional Golfers' Association of America's (PGA) flagship event in 2005, made five birdies in his last nine holes to card a three-under 69.

That took the left-handed American, now 50, to five under par for the tournament in South Carolina.

Mickelson is joined there by South Africa's Oosthuizen, who had squeezed ahead of Mickelson and was destined for the outright lead until he bogeyed the last hole.

Oosthuizen's first dropped shot of the round meant he carded a four-under 68, tied for the best round of the day.

Brooks Koepka, a two-time winner of the PGA Championship and a four-time major champion, is one shot off the lead, thanks largely to eagles on the seventh and 11th holes.

Japan's Masters champion Hideki Matsuyama is one shot further behind, having also shot 68, and is joined in a tie for fourth place by two more South Africans, Branden Grace and Christiaan Bezuidenhout.

Corey Conners of Canada, the leader after round one, is now at two under par for the tournament and tied for seventh place, having dropped three strokes today.

American Bryson DeChambeau is one shot behind Conners in a seven-way tie for 12th.

The back nine and particularly the final five holes played very tough, with Rory McIlroy - who won this event at Kiawah Island in 2012 - among those to drop shots.

Having fought his way back to even par, McIlroy bogeyed each of the closing three holes to fall eight shots off the lead.

Of the top 10 players, only England's Paul Casey and South Korean Im Sung-jae - both two under for the tournament - played the last five holes to even par or better.

The second men's major of the year, the PGA Championship is due to conclude on Sunday (May 23).