As Liberton's Anthony Blaney went out in the fifth round after morning exits for Willem Kerr (Craigmillar Park), Euan Bowden (Glen) and Ewen Donaldson (Craigielaw), Forrest won both his matches despite the fact he wasn't firing on all cylinders.
He was only one up at the turn in the morning before chalking up a 4 and 2 win over Ayr Belleisle's Ciar Porciani.
Then, in the fifth round, the Scottish Under-16s champion from Craigielaw came back from being one down at the turn to beat Dumfries & County's Liam Johnston, one of his best mates on the circuit on the last green.
In the latter, Forrest hit a 6-iron to 15 feet for a hole-winning birdie at the 15th only to undo that good work when he three-putted the next hole to go back to all square.
But he made amends for that in style – holing a 40-foot birdie putt to win that before a half in par at the last was good enough to take Forrest through to a last-eight clash with Alasdair McDougall, who has entered the event out of Elderslie but is also a member at West Kilbride.
"The putt at the 17th got me pumped up and I had to try and calm myself down on the last tee," admitted Forrest.
"My golf was a bit scrappy today, to be honest, and my short game probably saved me. I got up and down well and also holed some good putts."
Asked what it felt like playing such an important match against one of his friends, Forrest, who is coached by Colin Brooks at the Braid Hills Golf Centre, added: "You've just got to try and put something like that to the side."
Blaney, a credit to his club during the week, went out in the fifth round after having the tables turned on him by Pollok's Conor O'Neil.
Blaney had been getting off to flying starts in his earlier matches but, on this occasion, found himself six down at the turn as his opponent went out in three-under-par.
As O'Neil lost his concentration for a spell, 17-year-old Blaney got it back to four at one point but, ultimately, had left himself with a losing battle and went down by 6 and 4.
"I've exceeded my expectations this week so I'm happy enough," said Blaney, who is off to Holland on Sunday to represent Scotland in an under-16s event. Simon Fairburn, the Lothians junior champion, also went out in the fifth round after going down by 3 and 2 to Falkirk Tryst's Jamie Lynch.
The promising first appearances in the event of fellow 14-year-olds Kerr and Bowden came to an end in the morning's fourth round.
Kerr from Craigmillar Park lost the opening two holes and went down by 5 and 3 to tournament favourite Jack McDonald, who then beat Ewan Scott of St Andrews by 5 and 4 in the fifth round. "It's been a great experience for me and I'm already looking forward to next year," said Kerr, who has another four tilts at this title.
Bowden, who is in the same boat, went down fighting against Troon Welbeck's Callum Beveridge, who eventually went through by 2 and 1, while Donaldson was far from disgraced in losing 2 and 1 to Alva's Lawrence Allan, who pulled off a real shock later in the day as he beat Scott Gibson (Southerness).