Kilmarnock goalkeeper Alan Combe was sidelined through injury, but the home side rarely threatened Harpur's goal.

Airdrie made two changes from their weekend victory against Alloa, with Marc Smyth and Paul Lovering restored to the first-team.

There was also a place in the starting line-up for on-loan midfielder Marco Andreoni.

Frazer Wright went close for Kilmarnock after five minutes, but his header from Willie Gibson's cross was well saved by Stephen Robertson.

Harpur was called into action moments later when he blocked Stuart Noble's drive and recovered the ball at the second attempt.

Kilmarnock broke the deadlock in the 24th minute when Hammill feathered Gibson's low free-kick into the back of the net from eight yards.

The visitors appealed unsuccessfully for a penalty when McKenna fouled Hammill, but referee Dougie McDonald ruled the offence took place outside the box and awarded a free-kick.

Kilmarnock were awarded a penalty when McKenna fouled Nish in the box and the forward converted the spot kick to double the visitors' lead.

Airdrie's misery was compounded when Smith saw red for a second bookable offence.

Kilmarnock now face cup holders Celtic in the fifth-round on Saturday.

Airdrie Utd: Robertson, Donnelly, Smyth, McKenna, Lovering, McDonald, Smith, Andreoni, Russell, Noble, Waddle. Subs: Hollis, Prunty, Watt, Holmes, Craig.

Sent-off: Smith, 83.

Kilmarnock: Harpur, Corrigan, Fowler, Ford, Frazer Wright, Hay, Gibson, Morgan, Hamill, Nish, Wales. Subs: Rascle, Locke, Invincibile, Fernandez, Bryson.

Goals: Hammill, 25. Nish (pen) 37.

Referee: D McDonald

Att: 3,258