The 18-year-old midfielder headed home 30 seconds from time to force extra-time and then scored a superb solo goal to clinch victory.

Welsh Premier side Carmarthen were 2-0 up after 51 minutes when Danny Thomas added to Paul Fowler's opener.

Kevin Campbell reduced the deficit with 25 minutes to go before substitute McDonald emerged as the unlikely hero.

Match report: BBC Wales' Ian Gwyn Hughes

His first goal was a simple header into an empty net after latching on to a chip ahead from Kevin Cooper following Chris Gunter's break down the right.

But he showed superb balance and poise on a soggy pitch for his second as he rounded home goalkeeper Neil Thomas after a 20-yeard run.

Cardiff, who lost 2-1 to Carmarthen at the same stage of the competition last season, started brightly with a side made up of reserve and youth team players.

Midfielder Cooper hit the post and striker Iwan Redan came close to opening his Bluebirds account on his full debut.

But Carmarthen responded well, and after shooting into the side netting minutes earlier, Fowler put the home side ahead with an 18-yard snap-shot which found its way through a crowd of players.

Cardiff goalkeeper David Forde, another debutant, then made a terrific save to deny Nathan Cotterrall before half-time.

But the home side doubled the lead just six minutes of the second-half as Danny Thomas broke down the middle to score against his former club.

Cardiff's revival began with 25 minutes to go as Campbell poked the ball in with his head after Bradley Middleton had headed across goal from a high ball into the box.

Substitute Andrea Ferretti missed a great chance to bring Cardiff level when he shot wide, while Forde again came to Cardiff's rescue by pushing Kaid Mohamed's shot onto the post.

Carmarthen had other opportunities to wrap the game up , and those misses came back to haunt them when McDonald struck in the 90th minute and then 13 minutes into extra-time.

Carmarthen almost found an immediate reply as Cardiff went to sleep at the back, but Forde was on hand again to frustrate the home side and secure the Bluebirds' passage into the semi-finals.

Carmarthen Town: Neil Thomas, Terry Green (Liam Hancocks 68), Paul Keddle, Chris Thomas (Colin Loss 75), Mark Dodds, Tom Ramasut, Danny Thomas, Sasha Walters, Paul Fowler (Mattie Davies 101), Kaid Mohamed, Nathan Cotterall.
Subs Not Used: Ashley Williams, Kieran Davis.

Booked: Mark Dodds, Paul Keddle, Chris Thomas.

Goals: Paul Fowler (31), Danny Thomas (51).

Cardiff City: David Forde, Chris Gunter, Matthew Smith, Danny Blake, Lloyd Evans, Bradley Middleton, Willo Flood, James Simmonds, Kevin Campbell (Andrea Ferretti 75), Iwan Redan (Curtis McDonald 75), Kevin Cooper (Kyle Bassett 105). Subs Not Used: Scott Allison, James Simpson.

Booked: Bradley Middleton, Danny Blake.

Goals: Kevin Campbell (65), Curtis MacDonald (90, 103).

Referee: Mark Whitby (Penallergeor).