Eintracht Frankfurt benefitted from the absence of VAR in the Europa League as they eliminated Benfica via the away goals rule following a 2-0 home win on Thursday.
Filip Kostic's goal from an offside position was allowed to stand in the 36th minute and Sebastian Rode levelled the tie at 4-4 just over 20 minutes from the end to send the Bundesliga side through to the semi-finals.
Benfica had established a strong position with a 4-2 victory a week ago but were unable to muster a response after the linesman's mistake in the first half left their lead looking tenuous.
Substitute Eduardo Salvio hit the upright with a late chance to put the Portuguese outfit back into the box seat, but Frankfurt held firm to book a final-four meeting with Chelsea.
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Frankfurt vs. Chelsea
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Benfica defended stoutly for much of the first half and only an officiating error eroded their two-goal advantage.
Replays showed wing-back Kostic to have been comfortably offside when he turned in the rebound from a Mijat Gacinovic shot that struck the base of the left post.
Bruno Lage's protestations resulted only in the Benfica head coach being sent to the stands before the break.
His team briefly displayed a greater sense of urgency after the interval as Haris Seferovic worked Kevin Trapp with a close-range header.
The hosts, though, always appeared a threat going forward and the crucial strike eventually came from an unlikely source, midfielder Rode side-footing into the bottom-right corner from the edge of the area.
The entire complexion of the contest almost swung back into Benfica's favour towards the death, but winger Salvio could only find the outside of the post from Alejandro Grimaldo's cross as Benfica fell narrowly short of advancing.