The Senegalese striker set up Ross McCormack’s equaliser before converting from the spot to complete a second half comeback that ensures a fourth round clash with Tottenham
Lee Clark’s side took the lead in the 36th minute when Wade Elliott latched onto Nathan Redmond’s pass into the penalty area and dispatched the ball into the net.
The Whites responded through a close range finish from McCormack in the 70th minute, before Paul Robinson handled in the box allowing Diouf the opportunity to cheekily chip the spot-kick into the net and seal the win for his side.
Jack Butland was rested from the Blues’ team with Colin Doyle replacing him in the starting line-up, which allowed young keeper Nick Townsend the chance to appear on the bench for the first time in his career. Elsewhere Marlon King was recalled to the team in place of Callum Reilly.
Neil Warnock made seven changes to his team that lost to Barnsley at the weekend. The Whites were without Michael Tonge and Ross Barkley who were both ineligible and Luciano Becchio who missed the game due to illness.
The game started quietly as both sides wasted possession, however, the hosts sprang to life and almost took the lead as Marlon King and Wade Elliott combined on the edge of the area. The former Burnley man couldn’t quite find the target, dragging his effort wide of the post.
Birmingham looked to hit Leeds on the counter attack, with King forcing Jamie Ashdown into a good save. However, minutes later Clark’s side took the lead as Redmond raided down the left and his pinpoint cross found Elliott who made no mistake volleying home from close range.
The Whites should have responded immediately as they finally supplied a decent ball into the area but Luke Varney somehow missed the target with a header, nodding Diouf's cross wide with the goal gaping from six-yards.
After the break Warnock’s side upped the ante and came close to getting an equaliser through Rodolph Austin who drove at the Blues’ defence and fired an effort from just outside the area which forced Doyle into a good save.
The visitors were dominating possession much like they had in the first half and finally got their reward for their persistence as McCormack equalised from close range, converting Diouf’s knockback into the six-yard box after Byram and Paul Green had combined well down the right.
Leeds’ confidence was up after the goal and they could have taken the lead almost immediately through Green but Doyle was on hand to make an excellent save to keep the scores level.
However, this was only a brief respite as the Whites were awarded a penalty when Robinson was adjudged to have handled the ball in the penalty area. Diouf stepped up and duly scored with a Panenka chip over Doyle to give his side the lead.
Birmingham desperately tried to equalise late on but Nikola Zigic’s headed effort was cleared off the line by Lee Peltier and Ashdown made an excellent saved to deny substitute Peter Lovenkrands which was enough to secure the win for the visitors.
Leeds will now host Tottenham in the fourth round of the cup, with the match scheduled for Sunday 27 January at Elland Road.