Chelsea's quarter-final second leg victory at Stamford Bridge earned the Blues a 3-1 aggregate success.
Controversial referee Damir Skomina, whose performance in Arsenal's last-16 exit against AC Milan led to a three-match ban for Arsene Wenger, was at it again at Stamford Bridge as the Blues reached the last four for the sixth time in nine seasons.
Lampard opened the scoring after Skomina awarded a hotly-disputed penalty, one of several debatable decisions that included the first-half dismissal of Benfica captain Maxi Pereira.
The depleted visitors laid siege to the Chelsea goal at times and Javi Garcia set nerves jangling with an 85th-minute equaliser, but substitute Meireles blasted home on the breakaway to seal a barely deserved home win in what was arguably the Blues' worst performance under Roberto Di Matteo.
They gave away the ball so often, it was almost as if they were practising for being without it for the upcoming clash against Barca.
Chelsea have been a bogey team for the European champions down the years, but Pep Guardiola's all-conquering side will hardly be concerned about facing them again on the basis of tonight's performance.
After winning 1-0 in Lisbon last week, the Blues' task looked all more simple after captain Luisao and Jardel were ruled out for the visitors, who were forced to play Javi Garcia and Emerson as makeshift centre-backs.
But despite caretaker manager Di Matteo warning of complacency, Chelsea made the most sluggish of starts and needed desperate blocks from Lampard and John Terry to keep their opponents at bay.
They finally got going and Joan Capdevila had to put his own body on the line after an eighth-minute corner fell to David Luiz, who had shaken off an ankle injury to start.
Terry vented his fury at more slack Chelsea play as Benfica continued to perform like the side that had caused Manchester United so many problems in the group stage.
But after Oscar Cardozo was booked for going through the back of Luiz, Chelsea were gifted a penalty midway through the half.
Ashley Cole beat Garcia to a long ball forward and went down after a legal-looking shoulder challenge from the Benfica man, Skomina immediately pointing to the spot and booking Perreira and Bruno Cesar for protesting.
Artur got a hand to Lampard's penalty but could not keep it out.
Pablo Aimar joined Cardozo in the book before the latter almost equalised on the half-hour mark, Terry clearing his half-volley off the line after a superbly-worked free-kick routine.
Branislav Ivanovic was the latest player cautioned for tripping Nicolas Gaitan, and when Skomina got his yellow card out again for Pereira's foul on John Obi Mikel, a red followed.
Ramires incredibly became the seventh player cautioned before the end of the first half, which finished with Benfica boss Jorge Jesus almost confronting the referee on the pitch.
The visitors were predictably fired up at the start of the second half and only a superb Petr Cech save from Cardozo's first-time shot kept Chelsea ahead.
It should have been 2-0 in the 50th minute when Ramires somehow let Salomon Kalou's cross run under his foot with an empty net gaping.
Emerson got a block on Torres' finish after being bamboozled by the rejuvenated striker, who then flicked a rebound wide after Artur saved from Mata.
Cardozo was withdrawn for Nelson Oliveira, with Terry soon following for Gary Cahill and Gaitan for Yannick Djalo.
Djalo soon had what looked a goalbound shot blocked behind, while Kalou shot straight at Artur on the break as an increasingly-irate Jesus was ordered to calm down by Skomina.
The Benfica fans, who were ironically chanting the name of UEFA president Michel Platini, were almost silenced when Mata drilled wide.
Djalo nodded over after Cesar was replaced by Rodrigo, Kalou fired another shot wide and Oliveira shot straight at Cech.
Mikel was the first player booked in the second half after tripping Rodrigo before Raul Meireles came on for Mata.
Cech came to the rescue again to tip Djalo's header behind but was completely powerless to prevent a completely unmarked Garcia heading in Aimar's corner.
Benfica poured forward and Oliveira wasted a great chance to dump Chelsea out before Didier Drogba came on for Torres to help see the game out, which Di Matteo's men did in stoppage-time when Meireles ran 40 yards and drilled home, the visitors' bench already on their feet in collective fury.