Birmingham City 2 Newcastle United 3 | OneFootball

Ethan Laird put the hosts ahead inside the first minute at St. Andrew's @ Knighthead Park but Willock levelled matters in a thoroughly absorbing opening to the contest.

Callum Wilson then prodded home on his first start of the season to give Newcastle the lead before Tomoki Iwata's stunning drive made it 2-2 before half time.

In the end it took another effort from Willock in the final ten minutes to settle the contest and send Eddie Howe's side through to the fifth round after a committed, disciplined display against the League One side, who made it far from easy for United.

Howe made nine changes to his team following Wednesday's 2-0 Carabao Cup semi-final second leg win over Arsenal, which sealed their place in March's final at Wembley with a 4-0 aggregate scoreline.

Another final berth in the most famous domestic cup competition of all is one of his side's aims this term but this was a tricky-looking tie to navigate, with Chris Davies' men currently sitting top of League One and having lost just twice in the third tier all season.

And it took less than a minute for the Blues to make it clear this would not be an entirely straightfoward evening. Their opener after 44 seconds was credited to Laird, but his shot took a hefty deflection off Wilson after the defender had latched on to Keshi Anderson's knockdown in front of Nick Pope's goal.

United looked to respond, with Osula teeing up Wilson for an effort that was well blocked by Christophe Klarer, but it was Pope - on his first start since December's Premier League defeat at Brentford - who was called into meaningful action before his opposite number Bailey Peacock-Farrell, producing a fine save at full stretch to repel Anderson's volley.

But just before the halfway point in the half, Newcastle levelled. It was the first time they had really threatened Peacock-Farrell's goal. Wilson's deep cross was turned back by William Osula and Willock's effort looked to have been clawed off the line by the hosts' keeper before he lashed over the loose ball - but the eagle-eyed assistant referee indicated that the ball had crossed the line, granting the Magpies an equaliser.

Just a few minutes later, they moved ahead. Osula brought down Pope's long ball magnificently and saw his shot beaten away before Sean Longstaff played in Tino Livramento on the right. His low cross found Willock, who couldn't turn it home, and then Osula somehow failed to apply the finishing touch too. Fortunately for those in black and white, Wilson was there to prod home the bouncing ball and quieten the home support.

Chances came and went for both City and United, with the impressive Willock seeing a goal ruled out for offside after tucking away Dan Burn's flick-on and Jay Stansfield planting a header wide of the target; an opportunity that really should have brought another leveller. Davies' men found one before the break, though, in wonderful style - Iwata's sensational first-time half-volley from 25 yards out was utterly unstoppable, and was still rising even as it connected with the roof of Pope's net.

Sandro Tonali was introduced in place of captain Bruno Guimarães at the interval and they began the second period with a greater degree of fluency, after Birmingham's flurried attacks had made the first half an entertaining spectacle. Ex-City defender Burn and frontman Wilson were then withdrawn, with Fabian Schär and Jacob Murphy - two of the stars of United's memorable midweek win - sent on to add some fresh impetus.

A lengthy stoppage due to an injury sustained by Marc Leonard stymied the game's momentum, with a Murphy snapshot the only real sight of goal until the final, fractious 20 minutes. With nine minutes remaining, Howe's men finally found a third, and it was a deserved one for Willock. Sean Longstaff's attempted cross was charged down but the former Arsenal man picked up the pieces, taking aim from a tight angle and squeezing his shot through Peacock-Farrell's legs to settle the tie.

There was still time for Pope to make another vital intervention, scrambling an attempt away following a corner, and for Murphy to go close to adding a third when he rounded Peacock-Farrell and saw his shot cleared off the line by Ben Davies. But this was - just about - a deserved win for the Magpies against spirited, energetic opposition, and their dream of reaching a second cup final this season is still very much alive.