Britt Assombalonga scored twice in Middlesbrough's 5-0 win over Peterborough in the FA Cup third round
Britt Assombalonga scored eight minutes before full-time to give Middlesbrough victory at Birmingham and strengthen Boro's position in the Championship play-off spots.
Boro led before half-time when Lewis Wing converted at the back post following good work on the right by Dael Fry.
Che Adams fired in Blues' equaliser but Boro's response was almost immediate, with substitute Assombalonga finishing well from Wing's pass.
Blues' first defeat in six league matches was only their second at home this season - and kept them four points adrift of the top six.
Middlesbrough, meanwhile, are fifth and only four points behind the top two.
Midfielder Wing impressed as a substitute in the 5-0 thrashing of Peterborough in the FA Cup seven days earlier and was rewarded with a start by boss Tony Pulis in place of on-loan Rajiv van La Parra.
And he followed his stunning long-range strike against the League One side with another goal and an assist at St Andrew's.
Before Wing's opener, Boro had looked like being frustrated in the first half when Blues full-back Kristian Pedersen made goal-line clearances to deny Jordan Hugill and Fry.
At 1-0, home centre-back Harlee Dean headed against the post and Adams' appeals for a penalty were met with only a yellow card for diving.
Adams combined with strike partner Lukas Jutkiewicz to equalise, only for Assombalonga to net Boro's winner from a counter-attack.
Birmingham manager Garry Monk: "It was the classic sucker punch. When you see the team sheet with four defenders and five midfielders you know they're going to be a difficult team to break down.
"I thought we were fantastic in the second half and the lads showed their character. We were the team in the ascendancy and using the ball a lot better.
"But then Middlesbrough scored almost straightaway and it was a simple situation we should have dealt with better. It was a harsh outcome for the players because I think they deserved something."
Middlesbrough boss Tony Pulis: "To come in just 1-0 up at half-time was a travesty. We should have been out of sight. We were by far the better team. I know we should have had two penalties so instead of being one up, we should have been three up at least.
"In the second half, they played very direct and we had to defend - and we did that ever so well.
"They've got a great goal and there were one or two other scares. But it was a great goal from Britt, too - he gets his chance and he finishes - and I think we deserved to win. Winning so well away from home shows the character of the team."
