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Andy Robertson scored a fine double as Dundee United beat Kilmarnock in a seven-goal Scottish Cup fourth-round thriller at Tannadice.

The Scotland Under-21s full-back fired the opener after 27 minutes.

Two goals in two minutes shortly after the break from Darren Barr and Chris Johnston put Kilmarnock ahead.

But, after a Stuart Armstrong equaliser, Robertson put United ahead, Brian Graham scored a penalty and Gary Mackay-Steven settled the game.

United stretched their unbeaten run to six games as they inflicted a fourth straight defeat on their unlucky Scottish Premiership rivals.

Kilmarnock, for whom Reuben Gabriel made his debut days after demanding a transfer as part of four changes, had been denied the services of on-loan striker Michael Gardyne against his parent club.

Yet they started brightly and Kris Boyd had a shot saved by Radosław Cierzniak then a Johnston effort deflected just wide off John Rankin.

However, Robertson pounced to fire the 27th-minute opener slightly against the run of play after a fine ball into the box was flicked on by Nadir Ciftci.

Robertson's Under-21s team-mate, Stuart Armstrong, missed a couple of chances as United gained confidence from the goal, with Kilmarnock shaken out of their stride.

Kilmarnock manager Allan Johnston replaced Sean Clohessy with Mark O'Hara at the break in an attempt to tighten his defence.

It not only steadied the visitors at the back, but defending higher up the pitch increased their threat at the other end.

However, they were given a helping hand in their bid to square the match when Cierzniak spilled the ball as he rose to collect a cross three minutes into the second half and Barr hooked in his fourth goal of the season.

An inspired Kilmarnock came close to taking the lead when Rory McKenzie's curling drive flew just wide.

Their second goal was only delayed until the 50th minute, when Mark Wilson failed to head clear and Johnston bent a right-foot drive into the top corner of the net.

United nearly grabbed the equaliser when Rankin's cross almost caught out Craig Samson, before the goalkeeper managed to claw the ball clear.

Kilmarnock continued to look the more threatening and Boyd and Johnston both missed opportunities to hit the target.

It was United, though, who struck next, with Ciftci using his strength to push aside O'Hara to set up Armstrong for the finish with Kilmarnock calling for the game to be stopped for an injury to Gabriel.

Ciftci was forced off the field shortly after with what looked like a hamstring injury to be replaced with Graham.

Boyd shot straight at Cierzniak before Graham came close to scoring with virtually his first touch, his header clipping the post from a Robertson cross.

Samson looked at fault when the United left-back's next contribution was a shot that was on target but looked fairly easy meat for the goalkeeper until it bounced over his body for the defender's fourth goal in three games.

United were now rampant and a powerful Gavin Gunning free-kick was parried by Samson before Gabriel pulled down Ryan Gauld and, although the Nigeria midfielder escaped a second yellow card, Graham slotted home the penalty.

The home side quickly ended any thoughts of a fightback when Mackay-Steven pounced on a defensive error to score again with six minutes remaining.