Scotland's Liam Johnston revealed he was blissfully unaware of his opportunity to card only the second 59 in European Tour history after setting a blistering pace at the Portugal Masters.
At the same venue where Oliver Fisher made history in 2018, Johnston went to the turn in 31 before firing five birdies in his next seven holes to leave him needing an eagle-birdie finish to break the magical 60 barrier.
The 27-year-old from Dumfries found the green in two on the par-five 17th, but left his eagle attempt short of the hole and was unable to find an unlikely two on the difficult par-four 18th.
A 10-under-par 61 was Johnston's best on the European Tour by six shots and gave him a one-shot lead over France's Julien Guerrier on a day of typically low scoring at Dom Pedro Victoria Golf Course in Vilamoura.
Pre-tournament favourite Tommy Fleetwood carded a three-under-par 68, which included an uncharacteristic hooked drive into the water on the 18th, to share 21st in a group along with defending champion Steven Brown.
Jason Scrivener the best of the Australian in a tie for 29th at two-under.
His 69 included five birdies and three bogeys.
Fellow Australians Jake McLeod and Zach Murray have work to do to make the cut after rounds of one-over 72 while Maverick Antcliff (74) and Wade Ormsby (75) are well off the pace.
But the day belonged to Johnston who had 10 birdies in a bogey-free round.
"I had no idea," Johnston said when asked if a 59 had been in his sights.
"I thought it was a par 72, but I was obviously trying to hole the putt on 17 for eagle but left it a little bit short before making a nice par on the last.
"I wouldn't say I felt a round like this has been coming, but I have been playing well."
Guerrier carded nine birdies in his bogey-free 62, with England's Laurie Canter two shots further back after a 64.
Fleetwood's round contained five birdies and two bogeys.
"The back nine was probably the best I've hit my irons for a long time," Fleetwood said.
"Overall I was happy.... The course is a different test to what it's always been."