The Royals started the game brightly, with both sides whipping in crosses and testing each other out. The first real chance of the game came after 12 minutes with a West Brom free kick whipped over the bar from Chris Brunt on 12 minutes.
Cox had made a bright start in an unfamiliar role playing as the lone striker, with Rakels providing the most support drifting in from the left hand side.
Great play from Hal Robson-Kanu on the right hand side won a corner around the 25 minute mark, with the set piece eventually falling to captain Paul McShane, whose shot went just wide.
Ben Foster was then forced to make a save from a Robson-Kanu effort shortly before half time.
0-0 at half time. A good first half.
West Brom failed to register a single shot on target in the first half, but still opened the scoring through West Brom captain Darren Fletcher. Sandro played the ball to him, who got the ball out of his feet and stuck the ball into the bottom corner, with nobody closing him down. 1-0 to West Brom.
The Baggies tried to turn the screw, again testing Al-Habsi shortly after scoring but they'd fouled in the build up.
Simon Cox was then replaced by Matej Vydra, who didn't get a touch of the ball before Paul McShane equalised from a Oliver Norwood free kick. 1-1.
The game went end to end, before Reading took the lead through another Norwood free kick, this time Michael Hector meeting the delivery.
West Brom began turning the screw on a nervous Reading side, with the Baggies unable to score on the back of two golden opportunities, most notably Fletcher missing from 4 yards.
West Brom had another opportunity in the final minute of the game after Hector gave away a free kick on the edge of the box. Pritchard's free kick was whipped into an offside player.
Robson-Kanu had a golden chance to put the icing on the cake when put through one on one, but conspired to miss. That wasn't the last action of the game, with the substitute Lucas Piazon putting the icing on the cake in the dying seconds of the game. 3-1 to Reading.
Reading saw the game out and end it as the outright top scorers in the FA Cup so far this season.