MK Dons' Papa John's Trophy run came to an end as Sunderland put three past a youthful Dons side.
The visitors went ahead in unfortante fashion as Diamond's cross took a deflection off Dean Lewington before wrongfooting Lee Nicholls.
The Dons dominated large spells of the second-half, however, two quick-fire Sunderland goals from Aiden McGready and Charlie Wyke took the sting out the tie as The Black Cats progressed into the Semi-Final.
Russell Martin made four changes from Saturday's win over AFC Wimbledon, with 17-year-old Lewis Johnson handed his second start of the season.
Lee Nicholls, Lasse Sorensen and Matthew Sorinola also came into the side, while there was a place on the bench for first-year scholar Charlie Smith.
After an opening five-minutes where both sides exchanged possession, it was the visitors who had the first chance of the game. Aiden O'Brien was on the end of Lynden Gooch's pass, but his effort hit the side netting.
At the other end, Harvie flicked a cross into the path of Mason - his effort was blocked by Tom Flanagan.
The breakthrough came for Sunderland with 12-minutes on the clock as an unfortunate deflection off Dean Lewington founds its way past Lee Nicholls. To their credit, it was an explosive counter attack from Lee Johnson's side - with Diamond's cross finding the touch off Lewington. 0-1.
The Dons looked for an immediate reply through Joe Mason who was on the end of Fraser's deflected effort. Remi Matthews was equal to his strike and was also on hand to smoother any rebounds.
Sorinola won then won the ball inside Sunderland's as the Dons looked to level proceedings before the break. He switched play to Harvie who found the run of Joe Mason with his driven cross. The striker met the delivery with perfect timing but was unable to keep his effort down.
The equaliser looked as if it would fall with the first-half drawing to a close. Mason played a clever flick into the path of Johnson and after the youngster beat his man looked for the run of Fraser. He was denied a tap-in by a last-ditch tackle from O'Nein meaning Sunderland led at the break.
With no changes for either side at the break, the second half was underway, and it was Martin's side who had the first chance of the half. Lewis Johnson found himself on the end of Fraser's pass - his dangerous cross was met by Matthews.
Fraser, who was coming into his own, then evaded two defenders before threading in Mason. His shot across goal was once again saved by Matthews.
Sorinola then almost netted his second in as many games as the Academy Graduate dropped a should to cut inside and create room for a shot. His curling strike took a deflected and ended narrowly wide of the post.
Nicholls was then called into action twice within quick succession. Firstly, he turned behind from Neil's strike before claiming the resulting corner.
Matthews proved to be the thorn in the side of MK Dons as he pulled off yet another impressive save. The chance was created after another free-flowing move involving Jerome, Mason and Fraser. The Scot played a clever one-two with Jerome before firing across goal - Matthews was down quickly to deny.
Having soaked up the pressure, Sunderland soon doubled their advantage through substitute Aiden McGeady. The experienced winger was on the end of Sorensen's clearance, and after taking a touch to set himself, his driven effort found the back of the net. 0-2.
Two quickly became three as Charlie Wyke headed home from a corner with 10-minutes left to play. 0-3.
The two sides meet again this Saturday with three points on the line.
MK Dons: Nicholls, Lewington (Smith 80), Fraser, O'Hora, O'Riley (Freeman 80), Sorensen, Mason, Harvie, Thompson (Gladwin 60), Sorinola (Davies 80), Johnson (Jerome 60).
Subs: Fisher, Walker.
Booked: Harvie, Gladwin.
Sunderland: Matthews, Flanagan, Willis, Power, O'Brien, Gooch, O'Nein, Scowan, Diamond, McFadzean, Jones.
Subs: Patterson, Maguire, Wyke, Leadbitter, Neil, McGeady, Younger.
Goals: Lewington (OG) (11), McGeady (76), Wyke (80).
Referee: T. Brammall.