El Ahmadi (pictured) opened the scoring after 20 minutes from close range after a blunder from PSV defender Dirk Marcelis and Giovanni van Bronckhorst doubled the lead with a marvellous long drive 14 minutes later.
League leaders PSV put Feyenoord under pressure and created a handful of chances with Ola Toivonen twice being denied by goalkeeper Rob van Dijk.
In the final minutes, El Ahmadi sealed the victory, which was PSV's first defeat in a competitive match since March 15 when they lost 1-0 at Feyenoord.
Ajax Amsterdam came from behind in extra time for a 3-2 win over NEC Nijmegen, who finished the match with nine men.
Netherland's midfielder Demy de Zeeuw put Ajax ahead two minutes before the interval when he struck from close range but a diving header from Bjorn Vleminckx in 83rd minute sent the quarter-final into extra time.
Rens van Eijden scored Nijmegen's second three minutes into extra time but Toby Alderweireld and Siem de Jong turned the tide in favour of the home side.
Nijmegen's Jeroen Heubach and Jeffrey Sarpong were both sent off after a second booking.
Twente Enschede cruised to a 4-0 win at Sparta Rotterdam with Blaise Nkufo scoring twice.
Second division side Go Ahead Eagles Deventer knocked out top-flight NAC Breda with a stunning 2-1 away win.
