Tony Pulis' visitors had the better of a quiet first half but were made to pay for their profligacy shortly after the break and were finished off when Claudio Yacob saw red
Aston Villa booked their place in the FA Cup semi-finals thanks to a 2-0 victory against 10-man West Bromat Villa Park, but not before a section of fans invaded the pitch ahead of the full-time whistle.
Fabian Delph opened the scoring six minutes into the second half, and after Claudio Yacob was sent off at the 80-minute mark Scott Sinclair broke down the left to put the result beyond any doubt.
Villa were also reduced to 10 men in stoppage time when substitute Jack Grealish picked up a second yellow card for diving, and the end of the match was delayed as supporters prematurely flooded onto the pitch.
They were forced back to the fringes for the final moments but ran back onto the field of play in their hundreds when the match was finally brought to a close.
The victory was Villa's second against the same opposition in the space of four days. It was Christian Benteke who netted an injury-time penalty to hand Villa a much-needed 2-1 Premier League triumph on Tuesday, but the Belgium international striker was absent when the teams reconvened at Villa Park in the cup.
Tim Sherwood's team lacked attacking impetus in Benteke's absence and West Brom had the better of the first half.
Saido Berahino scored West Brom's goal in midweek and he forced Shay Given - recalled for cup action in the Villa goal - to beat away a sixth-minute drive.
Craig Gardner was the provider on that occasion and the West Brom midfielder delivered a superb right-wing cross shortly afterwards, but striker Brown Ideye could not find the back of the net despite his recent good form.
Given kept out a 32nd-minute header from Ideye, Craig Dawson supplying the right-flank ammunition on this occasion, before the Republic of Ireland veteran arched backwards to tip Chris Brunt's deflected long-range effort over the crossbar.
The hosts failed to offer a genuine first-half test to West Brom's Boaz Myhill until the cup goalkeeper gathered Sinclair's low centre at the end of a darting dribble in stoppage time.
But Villa emerged for the second period displaying far greater intensity and were soon rewarded in the 51st minute.
Charles N'Zogbia cut in from the right, drawing West Brom defenders towards him, before picking out Delph to drill a low left-footed shot beyond Myhill.
Tony Pulis' team should have been back on level terms six minutes later, but Joleon Lescott mistimed his jump to head wide at the back post after a corner from the left looped up off Villa's Tom Cleverley.
Given had to be out sharply to thwart Berahino with an hour played, while Myhill reacted similarly when Gabriel Agbonlahor and N'Zogbia combined to send Sinclair racing into the box.
The on-loan Manchester City winger then spurned the chance to give Sherwood's men breathing space as he dragged wide after Agbonlahor's 70th-minute flick on.
Dawson did well to block from Ashley Westwood and Gardner almost put through his own as the better chances continued to fall against an increasingly frustrated West Brom.
But when Yacob was harshly awarded a second caution for an 80th-minute challenge on Agbonlahor the game turned, and West Brom were under-manned as Sinclair coolly strode forward on the break, cut back onto his right foot and curled home to settle matters.
It was Grealish who laid on that goal and it was certainly an action-packed cameo from the youngster.
Having picked up a yellow card for foulding Callum McManaman after 87 minutes, he then went down under an injury-time challenge from Lescott, only to see red when he got back to his feet after referee Anthony Taylor decided he had dived.
That would not be the final talking point, however, as scores of home supporters streamed onto the pitch before Taylor had called time.
They soon fled the scene of the crime but could land their club in hot water given that they, and many more, ran back to celebrate with the players, and in some cases confront the opposition, upon the final whistle.
