Arsenal's season is threatening to fall apart after Wayne Bridge hit an 87th-minute winner to earn Chelsea a 2-1 victory at Highbury and a place in the last four of the Champions League.
Bridge struck a cross shot beyond Jens Lehmann after a clever one-two with Eidur Gudjohnsen. His goal came two minutes after Ashley Cole had cleared Gudjohnsen's shot off the line with Lehmann beaten.
Bridge struck after Frank Lampard's 51st minute goal had cancelled out Jose Antonio Reyes's 45th-minute opener for the Gunners.
It clinched Chelsea's first win over Arsenal in 18 matches and a meeting with Monaco for a place in the final. It will be Chelsea's first European Cup semi-final appearance. Arsenal have still never progressed beyond the quarter-finals.
Bridge said after the match: "When Eidur's shot was cleared off the line, I wondered if it was going to be our night. Then Eidur played a great ball for me and I sidefooted into the corner of the net." He added: "We're going to get stronger after this." Lampard had drawn Chelsea level by clinically tucking away the rebound after the edgy Lehmann had failed to hold Claude Makelele's long-range shot. He said: "Even when we were behind at half time, we still believed we could win. We just grew stronger as the game went on." It was Claudio Ranieri's first win over Arsene Wenger's side as Chelsea boss.
He said: "The second half was a great performance. We stayed calm and kept our belief." Ranieri's half-time substitution - winger Jesper Gronkjaer for midfielder Scott Parker was a clever move too. Gronkjaer stretched the Arsenal defence and stopped Ashley Cole from pressing forward.
The scoreline emphasised Arsenal's sudden slump. The Gunners, who lost the FA Cup semi-final to Manchester United on Saturday, have seen their Treble hopes evaporate in five days.
Now they have only the Premiership title to play for and their nerves are showing - and Chelsea are waiting to pounce again.
Wenger's side are four points ahead of Chelsea with a game in hand. But they face tough games at home to Liverpool on Friday and away at Newcastle on Sunday.
Arsenal lacked physical power. They were too often out-muscled. They looked leg-weary and too many of their important players - Thierry Henry most of all - were below par.
As in the FA Cup semi-final, they paid for missing chances when they dominated early in the game.
Henry shot over from Edu's probing 11th minute pass and the French striker fired wide again in the 19th minute after Reyes' throughball split the Chelsea defence.
Robert Pires met Cole's 37th minute cross with a near post header. The move was a replica of his goal in the first leg, only this time his header hit the side-netting.
Reyes stabbed Arsenal ahead in the dying seconds of first-half stoppage time.
The Spanish forward reacted fastest after Henry knocked back Lauren's deep cross and Freddie Ljungberg touched the ball on. But it was not enough on a night when Chelsea paid back their recent defeats by Arsenal with avengeance.