Rubbing off each other, the duo carded matching five-under-par 67s, though they took different routes to sitting a shot behind the pacesetter, Frenchman Antoine Schwartz, in the latest event on the Arena circuit.
Locke was bogey-free after saving a courageous par on the final hole, where he drove it into the wispy bushes, then chipped out backwards and made his up-and-down with a putt from 12 feet.
“I played very steady today,” said the Paul Lawrie Golf Centre player, who finished as leading amateur in the 2018 Open at Carnoustie. “Just need to get my wedges closer tomorrow and it can be even better.”
Plant, the Silver Medallist at Royal Birkdale in 2017, ran up a triple-bogey 7 on the par-4 eighth hole but repaired that damage with a sizzling back nine of 30.
On a day when Schwartz claimed top spot with a bogey-free effort, Russian amateur Ivan Striganov also opened with a 67, as did American Kevin Rhoderick, Ireland’s Paul McBride and England’s Jake Ayres.
l Scottish duo Darren Howie (66-70) and Connor Wilson (65-71) shared fifth spot in the stroke-play qualifying for the South African Amateur Championship in Johannesburg.
They were joined in progressing to the match-play phase at Royal Johannesburg and Kensington Golf Club by fellow Scottish Golf squad players Stephen Roger, James Wilson and Stuart Easton, as well as South Africa-based Scot Aran Sinclair.