Birmingham fought off a late Cambridge fightback to win 3-2 and progress in the FA Cup.
Despite there being 40 places between the sides, Sky Bet League Two Cambridge might have fancied their chances beforehand after suffering only one home defeat this season.
But the game was effectively over before half-time thanks to Kai Wagner’s deflected effort, his first goal since signing from Philadelphia Union, and Kyogo Furuhashi’s clinical finish.
Cambridge could have gone ahead after half an hour when Dominic Ball’s cross was nodded down to Ben Knight, but he fired over the crossbar when well placed.
Instead the Championship side were ahead only a minute later. A fine block denied Furuhashi but the ball fell to Wagner, whose effort took a crucial deflection off Mamadou Jobe and flew in over goalkeeper Jake Eastwood.
Three minutes before half-time Furuhashi ended his 23-game goalless streak, winning possession from a dithering Zeno Ibsen Rossi before running clear and firing past Eastwood.
Furuhashi almost added another soon after the restart with a lofted effort which hit a post and went wide, before Knight was again guilty of blazing off target following good work by Louis Appere.
Lewis Koumas’ diving header forced Eastwood into a save as Birmingham looked to make things safe as the game entered the final 20 minutes.
The third goal came 11 minutes from the end through a simple goal as the hosts gave possession away and Tommy Doyle fed Marvin Ducksch, who rolled the ball into the far corner.
Cambridge responded immediately as Appere’s determined play helped work the ball to substitute Sullay Kaikai, who also found the far corner with a precise finish.
Kaikai scored a second a minute from time with an excellent thumping strike, but the Blues saw out stoppage time to progress to round four.