United face Aberdeen or Livingston away in the quarter-finals and Micky Mellon's side will have to improve if they are to go any further in the competition.

There was some drama before the game when Tannadice keeper Ben Siegrist picked up an injury in the warm up and had to be replaced by Deniz Mehmet.

The home side, who could only muster four substitutes, immediately unsettled their Premiership opponents and in the eighth minute striker Steven Doris fired over from a corner when he had time and space to be more considered with his finish.

United, with Pawlett and Clark also back in the starting line-up, slowly stirred.

In the 24th minute, striker Lawrence Shankland latched on to Jamie Robson's searching pass but Forfar keeper Marc McCallum blocked the close-range effort with his leg before saving a Clark header destined for the bottom corner minutes later.

In between those two United efforts Forfar had threatened again.

A long ball from Daniel Scally sent attacker Jordan Allan racing through the middle with Mark Reynolds and Ryan Edwards chasing but he snatched at his shot from the edge of the box and it sped wide.

Just before the break, after Roberto Nditi fouled Clark 25 yards from goal, and the former Rangers forward curled his free-kick off the junction of post and bar.

United flexed their muscle soon after the break.

Midfielder Calum Butcher's drive missed the target by a yard but moments later, from a well-worked corner, defender Liam Smith squared to Pawlett and from outside the box he fired past McCallum and in off the post.

The visitors grew in confidence and their superior fitness started to become a factor but they could not subdue Forfar and in the 75th minute substitute Scott Fenwick, on for Doris, headed over the bar from 10 yards.

Forfar had to draw on all their reserves in the final minutes.

In the 82nd minute United attacker Marc McNulty's angled-drive was cleared off the line by Murray MacKintosh, meaning the Taysiders could not relax until the final whistle.