Birmingham City 0, Oxford United 1 (Sercombe 120)

The tie had been on a knife-edge all evening, but just as a penalty shoot-out looked inevitable, the tireless midfielder popped up at the back post to head home.

It capped off a night where United had to be at their best defensively.

Goalkeeper Simon Eastwood made several good saves, while the visitors’ back four all had a strong night.

The underdogs had fewer chances, but the longer the game went on the chances of an upset increased.

And right at the death they added Birmingham City to Brentford and Bristol City as their third successive League Cup first round scalp and get themselves into Wednesday night’s second round draw.

United’s only change saw Aaron Martin, now recovered from a foot blister, replace the injured Chey Dunkley.

The centre back had the first effort of the game, nodding Chris Maguire’s free-kick tamely for Adam Legzdins to save.

U’s boss Michael Appleton emphasised the importance of the opening exchanges and he will have been relatively pleased with the underdogs’ start.

They saw plenty of the ball and City’s only early chance came from a set piece, when Paul Robinson fired over when a corner fell his way.

When United were opened up for the first time, on 20 minutes, it came via a mistake.

John Lundstram was mugged just outside his own penalty area, but Paul Caddis dithered when two teammates were poised at the back post.

• Oxford United were briefly down to ten men in the first half while Ryan Taylor's head was bandaged by Andrew Proctor

United cleared and quickly forged a shooting chance of their own.

A poor clearance from Legzdins led to Alex MacDonald being fouled 20 yards out, from where Maguire curled the set piece a yard over the crossbar.

United lost Wes Thomas to injury and their night almost got worse as City had their best spell of the half.

They were indebted to goalkeeper Simon Eastwood, who made two excellent saves in the space of three minutes.

First, the goalkeeper raced off his line to block a shot from James Vaughan after a ricochet had fallen kindly to the former Everton striker.

The lively Viv Solomon-Otabor, who had a spell on loan at Oxford City in 2014, was the next to be denied after he cut in from the left flank and drove a low shot from 15 yards.

Eastwood also made a smart parry to keep out a shot from the excellent Robert Tesche to keep the game goalless at the break.

United were unable to stretch Legzdins in the same way, but they emerged for the second half with more intent and within three minutes almost opened the scoring.

Maguire’s cross was nearly flicked in by Taylor at the near post and when MacDonald volleyed the ball back across goal it was a fraction too high for Sercombe to head home.

• Oxford United were backed by 1,155 travelling supporters

The game was much more even now, with the teams taking it in turns to attack.

Despite plenty of promising moments at either end, the two goalkeepers were relatively untested.

Blues boss Gary Rowett made nine changes, but with the game goalless he ended up throwing on Stephen Gleeson, Jacques Maghoma and Clayton Donaldson.

It was United who came closest to a late winner inside 90 minutes, though.

Kane Hemmings glanced a header against the woodwork, although the offside flag was up, while in the final seconds Sercombe had a stabbed shot smothered by Legzdins after a clever pass from Dan Crowley.

• Liam Sercombe is denied by Adam Legzdins in the 90th minute

The City goalkeeper made an even better save in the first half of extra time to keep out MacDonald, who had done superbly to control Maguire’s cross and beat his man.

At the other end Eastwood’s good night continued, tipping over a vicious free-kick from Gleeson.

The second period was characterised by tired legs and minds – and one mistake in the final minute set up the victory.

Legzdins did not deal with a deep cross from Skarz and Sercombe was on hand to nod into the empty net.

It came right in front of the travelling fans, who eruped in delight at another superb cup night.

Birmingham (4-4-2): Legzdins, Dacres-Cogley, Morrison, Robinson, Grounds, Caddis (Maghoma 78), Kieftenbeld (Gleeson 74), Tesche, Solomon-Otabor, Vaughan, Storer (Donaldson 78).

Unused subs: Kuszczak, Shotton, Shinnie, Fabbrini.

Booked: Storer.

Oxford Utd (4-4-2): Eastwood, Long, Nelson, Martin, Skarz, MacDonald, Sercombe, Lundstram, Maguire, Thomas, Taylor.

Subs used: Hemmings (Thomas 28), Crowley (Taylor 74), T Roberts (MacDonald 108).

Unused subs: Buchel, Welch-Hayes, Ruffels, Rothwell.

Booked: Taylor, Lundstram, Crowley, Skarz.

Referee: Darren Handley (Lancashire).

Attendance: 7,202 (1,155 visitors).