Leeds set up an FA Cup tie at home to Tottenham after coming from behind to dump out Birmingham 2-1 in their third-round replay.
Wade Elliott put Blues ahead shortly after the half-hour mark, just as he had done in the initial meeting which finished 1-1 10 days ago.
But once again Leeds levelled after the break, this time courtesy of Ross McCormack's first goal since September - an effort which had not looked like coming with 19-goal striker Luciano Becchio missing due to illness.
El-Hadji Diouf, celebrating his 32nd birthday, then quickly completed the turnaround, keeping his cool to chip home from the penalty spot after Paul Robinson had been penalised for handball.
It means Leeds will now host Spurs in the fourth round of the competition in a televised clash next Sunday, denying cash-strapped Birmingham the chance to pick up a welcome sum in the region of £200,000.
Brentford lined up a dream west London derby with FA Cup holders Chelsea thanks to a thrilling 2-1 victory over Southend.
But they had to sweat until Clayton Donaldson's 76th-minute winner after coming up against former goalkeeper Paul Smith in inspirational form.
The Shrimpers stopper pulled off at least a dozen top-class saves to repeatedly deny Uwe Rosler's side, who managed an incredible 30 shots.
Paul Hayes gave the hosts the lead just before the half hour when he volleyed home Lee Hodson's far-post cross from close range., but Paul Sturrock's side clawed their way back into the game after 68 minutes with a rare attack when Barry Corr headed home Ryan Leonard's cross at the far post.
The match-winner came with 14 minutes remaining when Donaldson's low drive took the slightest of deflections off Luke Prosser to finally beat Smith and set up a cup cracker at Griffin Park.