City were beaten by a late goal despite a dominant second-half display in the Midlands.
Hosts Birmingham City carved out a series of opportunities in the first period without enjoying the majority of possession, but found themselves hanging on after the interval.
Lee Tomlin struck the crossbar from a 25-yard free kick, but three minutes later Che Adams poached the winner when City switched off at a corner.
Jonathan Spector saw red in the 90th minute for a bad tackle on Tomlin, leaving Lee Johnson incensed that play was stopped with his team looking dangerous on the break.
The head coach made three changes to his side as City returned to action after two weeks away due to international fixtures.
Frank Fielding was fit to retake his place between the sticks having overcome a groin/hip problem, while Ivan Lucic leapfrogged Richard O’Donnell as the substitute goalkeeper.
Elsewhere, Marlon Pack and Bobby Reid earned recalls in a three-man midfield, with Joe Bryan switching to left-back as Scott Golbourne and Korey Smith dropped to the bench.
City would enjoy plenty of early possession in good areas, with Birmingham content to sit back and counter-attack despite being the home side.
Blues had not posed a threat until they came within a whisker of opening the scoring on 11 minutes.
Clayton Donaldson, who struck a hat-trick in the same fixture last season, turned inside on to his left foot and hit a low shot that beat Fielding but not his left-hand post.
As the ball bounced back off the upright, City were indebted to Mark Little for throwing himself in the way of Lukas Jutkiewicz’s follow-up.
The hosts are a team not short on physical prowess from set-pieces, but City had them worried from a pair of corners in quick succession.
First Tammy Abraham was only denied by a flying save from Tomasz Kuszczak, before Aden Flint glanced a header beyond the far post.
City were having more of the ball, yet Birmingham carried the greater threat from open play.
After David Davis side-footed wide from the edge of the box, Jutkiewicz escaped in behind.
Fielding did well to rush from his line and close down the angle, before Little again cleared crucially from Maikel Kieftenbeld with the City keeper still out of his goal, although the final shot was weak.
Welshman David Cotterill, who started his career at Ashton Gate, then produced a peach of a cross nodded only inches wide by the menacing Jutkiewicz.
Birmingham had enjoyed more than their fair share of chances, but City still looked bright with the ball and were only missing a final shot or cross to prise open their opponents.
One thing the visitors had not tried was the more direct approach, so Pack took matters into his own hands four minutes into the second period.
When Abraham stepped inside from the right touchline, the ball ran kindly into the path of the midfielder, who unleashed a rocket of a shot from 30 yards that needed tipping over from Kuszczak.
But much like the first half, having started on the back foot, the hosts then carved out a decent chance, as Cotterill collected a deep cross and teed up Jutkiewicz, whose goalbound shot was blocked by Flint.
The on-loan Burnley striker was a man on a mission. Still he kept finding the right positions, forcing Fielding’s best save to date with a powerful downward header from a Cotterill free kick almost too close for comfort for the keeper, who performed heroics to prevent it squirming beneath his body.
There may have been no goals, but this was engaging viewing with the game hanging in the balance.
City continued to probe with the ball. Gary O’Neil was influential in an unusually advanced midfield role while Pack pulled strings from deep.
By the midpoint in the second half, the visitors were dominant. The busy Paterson showed some neat footwork on the edge of the box to set up Tomlin for a shot that deflected behind for a corner.
Then Pack slipped the ball between defenders for Reid, who was denied at his near post by Kuszczak, before Bryan’s route to goal was blocked by another blue shirt.
City continued to press with increasing intent. Pack should have done better when he met a Bryan corner with a header wide from six yards, before Paterson and Tomlin combined to find Reid, whose low cross just evaded the lurking Abraham.
It was the sort of game that might take a piece of Tomlin magic to unlock the door.
That moment so nearly arrived 12 minutes from time. After Tomlin himself was fouled 25 yards from goal, City’s maverick number ten executed a free kick that looked destined for the top corner, only for Kuszczak to get fingertips to the ball and somehow touch it on to the crossbar.
The significance of that particular ‘what if?’ moment was amplified when Birmingham stole the lead from an isolated attack three minutes later.
Two substitutes combined, as Greg Stewart’s near-post corner was flicked on and turned in from close range by his fellow replacement Adams.
The goal did nothing to alter the pattern of play. Reid volleyed wide, Flint headed over when anything on target was a goal, and Abraham skied an instinctive chance from a low cross, but Birmingham survived despite the late dismissal of Spector for a crude lunge on Tomlin.