AND then there were three.

Dundee United became the third SPFL Premiership team to reach the quarter-finals of the William Hill Scottish Cup yesterday when they dismissed St Mirren in a bout between two sides who could have been forgiven for entering this match with the dented confidence of a derby winner venturing out for the figure skating final in Sochi.

United and the Saints have form in the way that the Krays had form. This was a welcome victory for Jackie McNamara, the first for United since December 21, ending a winless run of seven matches. St Mirren have only won once since battering United 4-1 in Paisley on Boxing Day.

This, then, should have been a close match between two struggling sides. It was only close on the scoreboard. United dominated but the margin of victory should have been greater than that brought about by Nadir Cifti's winner in the second half. That came after a first period in which Ryan Gauld's seventh goal of the season was cancelled by a Kenny McLean penalty.

The match was physically contested with six bookings. One of those was for Jim Goodwin, banned for two matches after elbowing Stuart Armstrong in the last encounter between the teams, who was given his routine yellow card amid a welter of booing. He was also involved in a clash with Gary Mackay-Steven that involved a hand in the face, but was considered innocuous by referee Craig Thomson. Eric Djemba-Djemba, St Mirren's headline signing, was also booked in a debut that started inauspiciously but gathered promise.

It was United, though, who showed more poise, pace and purpose. Gauld was intermittently threatening on his return after a much-needed break but it was the left-wing axis of the enterprising Andrew Robertson and the tricky Mackay-Steven who caused St Mirren and Jason Naismith, in particular, such consternation. It was a surprise that the score remained so close in the latter stages, and Radoslaw Cierzniak had to dive acrobatically to palm away a Steven Thompson header and thwart the visitors' hopes of a replay.

Both United goals almost inevitably came from excursions down the left flank. The first occurred when Cifti glided away from Djemba-Djemba after 21 minutes to slide the ball across goal for Gauld to divert it past Marian Kello. The second came amid a United siege when Mark Wilson pushed the ball wide to Mackay-Steven, who cleverly stood the ball up at the back post for Cifti to head home confidently.

Djemba-Djemba, wearing No.99, was as much good as a cone with a Flake stuck in it for the first goal but gained a foothold in the match and was an integral part of the only period when St Mirren looked likely to prevail in this cup tie. This came after the referee adjudged Wilson to have clipped McLean as he ran into the box. The St Mirren midfielder converted the penalty with ease.

Danny Lennon's side then had a period of comfortable possession even if they could not force a save of consequence from Cierzniak. However, they reached half time with hopes of a quarter-final place intact, despite enduring 45 minutes when Armstrong was unsuccessful with three strikes from good positions and Kello was superb in denying Mackay-Steven, when the United winger was clear on goal, and then Cifti, whose header lacked power and precision.

St Mirren, though, were blown away immediately after the restart. United started with such belligerence that the Paisley side would have needed Michael Caine on a white horse to marshal the defence. The almost constant battering yielded the winner, though United's carelessness in front of goal could have been punished if Thomson had been more emphatic with his header at the end of the match.

McNamara, though, praised the intensity of his side's performance, even if he freely admitted opportunities were scorned. "Six of the boys who started today are 21 or younger, so we are a young, energetic team and we will learn from playing against experienced players. The two centre halves - Souttar and Good - are only 20 and are playing against someone like Stevie Thomson who is a real threat aerially and who is clever in how he gets into positions," he said.

In truth, the United defence was never under sustained pressure and Lennon reflected: "We had a few players who certainly didn't reach the levels we know they can. Unfortunately for us, you don't get a second opportunity in this wonderful competition. I thought United had the upper hand in some of the personal battles."

Gauld was predictably more upbeat. "It's the best we've played in a while," he said. "Personally, I felt closer to my best. In previous games, I've been nowhere near what I expect of myself. There's still a good bit to go but I was happier with my performance. And there's no reason why we can't go on another run of wins. We've done it before and we can do it again."

United now face Inverness Caledonian Thistle or Stranraer for a place in the last four of the cup. St Mirren, though, need to regroup for a battle to avoid the relegation play-off place in the Premiership.