Johnny Russell scored a second-half hat-trick as Dundee United cruised into the fifth round of the Scottish Cup at the expense of Airdrie United.

John Rankin fired the 2010 winners ahead, with Scott Robertson adding a cool side-foot finish.

Russell lashed in a powerful strike 17 minutes after the interval and then headed home Paul Dixon's cross.

Gary Mackay-Steven powered in a shot before Russell struck again, with Ryan Donnelly replying twice late on.

The hosts had defeated Gala Fairydean 11-0 in the previous round and their bright start suggested they were in the mood to avenge a 2-0 defeat to United in the Scottish Communities League Cup earlier this season.

Jamie Stevenson saw his deflected effort from the edge of the area fly just over and from the resultant corner the Airdrie midfielder came close again, this time seeing his shot come back off the crossbar.

However, the Second Division side were soon undone on when former Diamond Mackay-Steven found Rankin bursting into the box and he fired powerfully past Connor Fairley.

Willie McLaren passed up a great chance to equalise when he volleyed straight at Dusan Pernis midway through the half and the Slovak was again on hand to deny Stevenson as Airdrie pressed forward.

It was the visitors though who were demonstrating the greater quality in the final third and in particular the combination of Dixon and Mackay-Steven down the left.

Dixon's low cross just evaded the on-rushing Russell and Jon Daly four minutes before the break but Airdrie didn't heed the warning and the second goal arrived just a minute later.

Mackay-Steven's excellent cut-back found Russell who laid the ball into Robertson's path and the midfielder expertly guided the ball into the top corner.

The second half followed a similar pattern as after a promising start to the half Airdrie conceded again. This time Russell found acres of room inside the area to control Willo Flood's cross before crashing an excellent finish into the roof of the net.

The gulf in quality was now starting to tell and the Scottish Premier League side added another two quickfire goals as Russell headed home another excellent Dixon cross before Mackay-Steven got the goal his performance deserved with an accurate shot across Fairley.

Russell completed his hat-trick as he slotted home the rebound after Fairley had blocked his first effort.

There was time for Airdrie to score twice as Donnelly first rounded Pernis and slotted into the empty net before heading in at the far post with the last touch of the game.

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