Richards settled a nervy confrontation against their npower Championship opponents when he rose to power David Silva's 73rd-minute corner past Alex McCarthy to send his side into a meeting with Sir Alex Ferguson's men.
City have met United in the last four once before, when the Blues came out on top 3-0 at Bramall Lane in 1926 before losing to Bolton in the final, which could in fact repeat itself.
This is a different era though. The hype will be massive as Roberto Mancini's men embark on their first semi-final since 1981 knowing victory over the club who have lorded it over them for so long will leave them one step away from the end of that 35-year trophy drought.
Reading began with confidence befitting a side boasting a seven-game unbeaten run, including a victory at Everton in the previous round, with skipper Jobi McAnuff sparkling around the Blues box and Noel Hunt not far off with an early header, but City quickly started to dominate.
Italian substitute Mario Balotelli's unselfish running earned the hosts the corner that brought their goal with Richards rising to meet Silva's curling effort and his header flashed past Reading goalkeeper Alex McCarthy.