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Celtic's Van Dijk, United's Paton and Dixon sent off

Ciftci scores penalty for United

United's Cierzniak saves Griffiths' penalty

Griffiths heads Celtic level

Teams will meet in quarter-final replay

Nine-man Dundee United held on at Tannadice to earn a Scottish Cup quarter-final replay with Celtic.

Referee Craig Thomson sent off Celtic defender Virgil van Dijk and United's Paul Paton in an early angry encounter.

Both teams hit the post before United's Aidan Connolly won a hotly contested penalty, which Nadir Ciftci scored.

Terrors defender Paul Dixon was sent off for blocking a goalbound shot and Leigh Griffths had his penalty saved before levelling with a great header.

The first eight minutes were scrappy, but the character of the game changed when, after a flurry of heavy challenges in midfield, Thomson showed the red card to visiting centre-half Van Dijk and, inexplicably, United's Paton.

Celtic skipper Scott Brown slid in on striker Ciftci, available for United after a successful appeal against a two-match suspension, and Van Dijk made an ugly challenge on Calum Butcher, who lashed out at the Dutchman while lying on the turf.

Paton arrived on the scene just after, but it was the holding midfielder and not Butcher who was shown a straight red card, along with Van Dijk, who had tussled with Butcher after the United player retaliated.

United striker Nadir Ciftci runs towards manager Jackie McNamara after scoring a penalty

Unless they are successful with an appeal, they will both miss next Sunday's Scottish League Cup final, the second of the three meetings between the teams in 14 days, not including the replay.

The incident further hampered the chances of witnessing any decent football on a muddy, rutted surface.

Indeed, the red cards apart, the first 40 minutes were forgettable.

During that period, a foul by Brown at the edge of his penalty box gave United a free-kick in a central position.

As seven Celtic players lined up to block the shot, three tangerine shirts peeled off the edge of the wall to open up Craig Gordon's goal, but Ciftci's tame shot slipped past.

Celtic, smarting from the midweek league defeat by St Johnstone, which was their first domestic loss of 2015, pushed forward but failed to penetrate the home defence until the 42nd minute when Stefan Johansen cracked a shot from 25 yards that Radoslaw Cierzniak did well to palm away.

United immediately had a chance of their own, with Butcher's poorly hit shot from 18 yards diverted against the Celtic upright by Ryan Dow.

The game swung to the other end and Johansen did brilliantly to squeeze between two United players to set up Anthony Stokes, who ought to have scored but instead rolled the ball, bobbling all the way, against the post.

Leigh Griffiths slams a penalty at the United goal - Radoslaw Cierzniak dived to his left to save it

In added time, Connolly weaved into the Celtic box and jinked past an unorthodox Nir Bitton tackle before crashing to the ground from what looked like only a glancing challenge by Stokes.

Thomson pointed to the spot and Ciftci tucked the penalty low just inside Gordon's right-hand post.

The second half began with now familiar drama, as James Forrest's goalbound shot was blocked by Dixon, who was almost on the goalline, with the aid of his arm.

The United defender was red carded, but Ronny Deila's side could not pull themselves level from the penalty as Cierzniak dived to his left to save Griffiths's well-struck shot.

The little Celtic striker almost made amends when he rose above Jaroslaw Fojut to meet Emilio Izaguirre's cross but headed just over.

In a febrile atmosphere, nine-man United tried to clear their heads as Celtic swarmed.

With half an hour remaining. United's Polish goalkeeper was a match for Stokes' blistering shot from the left after Forrest had run riot in a diagonal charge across the pitch.

But his save from substitute John Guidetti's low drive from a sublime Johansen lay-off was even more impressive, as United fought valiantly for a semi-final place after three defeats and a draw in their last four matches.

For all that, he was undone by a precise 71st-minute header by Griffiths, who timed his run perfectly to stay onside to meet Johansen's delicious clip into the box and direct the ball past the helpless goalie.

Celtic looked to deliver the killer blow, with Bitton and Johansen the most likely to create the decisive moment, and pressed forward relentlessly.

Butcher had the best of United's precious few second-half chances as he strode into the box, but had his shot saved by Gordon.

Cierzniak made a brave save at the feet of Guidetti to earn his team a replay at Celtic Park, which will determine who plays Inverness or Raith Rovers in the semi-finals. Hibernian and Falkirk will meet in the other last-four match.

Virgil van Dijk and Calum Butcher clash, resulting in the Celtic defender being sent off

Referee Craig Thomson sends off Paul Paton and Virgil van Dijk

Craig Gordon is at full stretch but cannot prevent Nadir Ciftci's penalty from crossing the line