Day, forced to wait overnight to take his shot as play was suspended on Thursday due to the threat of lightning after his tee shot landed in the right rough, left his wedge about nine feet from the hole at the ninth.
He then two-putted for a par that gave him a career-best 10-under 61 and a four-shot lead at Conway Farms Golf Club.
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Day was bidding to become only the seventh player to shoot a magical 59 on the PGA Tour and first since American Jim Furyk joined the exclusive club two years ago when he accomplished the feat in the second round of the BMW Championship.
The red-hot Day, in pursuit of world number one status and a fourth win in six starts, could leapfrog Rory McIlroy and Jordan Spieth to reach the top of the rankings with victory this week in the third of the PGA Tour's four concluding playoff events in the season-long FedExCup race.
Spieth, playing alongside Day, had a hole-in-one on Thursday, eventually going on to shoot 65 to sit four shots behind the Australian.
PGA Championship
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