Brentford's Marcelo Trotta opened the scoring only to have his effort cancelled out by Oscar just after halftime. Chelsea goalkeeper Ross Turnbull committed a foul in the area that led to Harry Forrester's go-ahead penalty, but Torres rescued the Blues with an 83rd-minute curling strike to keep the defending FA Cup champion alive.
The hosts settled into the game well, forcing their more-heralded counterparts on the back foot, and they went close to going ahead when a defensive mix-up between John Terry and Turnbull resorted in an indirect free kick inside the box that Forrester blazed just over the bar.
The Bees forward came close for the hosts again just before the half-hour mark, as he wriggled past three players on the left flank before cutting inside and unleashing a fierce drive over Turnbull's bar.
The breakthrough eventually came three minutes before halftime. It was Forrester again who stung the hands of Turnbull with a shot from range after dispossessing Frank Lampard, but the Chelsea keeper could only parry as far as Trotta, who made no mistake in drilling home the finish.
Benitez brought on Juan Mata for Marko Marin after the break as Chelsea hoped to find a way back into the tie, and it did not take the Blues long, as Oscar plucked a looping ball out of the air just outside the box before stepping past one defender and prodding a clever finish past Simon Moore.
Just when Chelsea looked to have a grip on the game, Turnbull brought down Tom Adeyemi as the Brentford substitute raced through on goal, and Forrester stepped and confidently tucked the penalty into the bottom left corner in the 73rd minute.
Brentford defended stoically to hold on to the lead but could do nothing about Torres' wonderfully shaped effort into the top corner in the 83rd minute after fine link-up play with Demba Ba, forcing another match between the two sides to determine which moves on to the fifth round of the tournament.
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