Essien, playing his first game since early March because of injury, drove home an angled drive in the 91st minute to put Valencia out in the most dramatic fashion.

Chelsea had looked dead and buried in the first half when Fernando Morientes had given Valencia a 2-1 aggregate lead.

But Andriy Shevchenko levelled and then Essien's last-gasp winner put Chelsea in dreamland.

The enormity of their task in the Mestalla Stadium had been underlined by the inclusion of midfielder Essien.

The Ghanaian international hurt knee ligaments during their 3-3 draw with Tottenham in the FA Cup quarter-final in early March and had not figured for the club since.

But his first contribution of the game was not the one coach Jose Mourinho had anticipated as he was booked for a crude challenge on Morientes.

The Blues were forced to defend in numbers in the 11th minute when Ricardo Carvalho gave away a corner. But Didier Drogba headed David Silva's cross to safety.

The English champions enjoyed large chunks of possession after Valencia's spirited opening but could not force the breakthrough they required.

They were awarded a free-kick when David Albelda fouled Michael Ballack some 30 yards out, but Andriy Shevchenko's effort was blocked easily.

In the 20th minute, John Terry headed a corner by Frank Lampard wide of the target as Chelsea continued to control proceedings.

Another corner found Ballack unmarked but his header went straight into the arms Santiago Canizares.

The Spaniards had been limited to a few counter-attacks and none of those had presented the Blues rearguard with any significant problems.

But all that was to change as Valencia delivered a real hammer blow to Chelsea's hopes of reaching the last four.

It was not as though Mourinho's side had not been warned. Morientes, who had missed the opening leg in London because of a shoulder injury, was clearly determined to set up a semi-final meeting with his former club Liverpool.

He was thwarted from giving the home side the lead in the 31st minute when he smashed a cross from David Silva against Petr Cech's right-hand upright.

But Morientes prevailed a minute later when he escaped his marker to volley home a superbly-delivered right-wing cross from Valencia full-back Luis Miguel.

The goal put Valencia ahead on aggregate but Chelsea could still save themselves by getting on the scoresheet.

However, it was Valencia who almost added a second in the 34th minute when another effort by Morientes was kept out by the legs of Ashley Cole.

It was going to take something special from Chelsea in the second half if they were to preserve their Champions League aspirations.

Mourinho made his first change of the night during the interval when Joe Cole replaced Lassana Diarra with Essien switching to right-back and Chelsea to a 4-3-3 formation.

Cole looked lively as Chelsea attempted to make progress down Valencia's right flank but Lampard was sloppy with his control and the home defence cleared the danger.

There was now greater urgency about Chelsea's play and when Albelda was booked for a foul on Drogba it presented the Blues with the chance to get back into the game.

The booking would rule Albelda out of the semi-final should Valencia win but their chances were handed a setback in the 52nd minute when they failed to deal with Lampard's free-kick and Shevchenko seized on the loose ball to equalise.

Shevchenko's goal put him just three behind Gerd Muller's total of 62 at the top of the European goalscoring charts.

The Ukrainian striker was now spearheading Chelsea's revival but it was his partner Drogba who caused more panic in the Valencia defence when he crossed into the six-yard box.

The home defence partially dealt with the problem but the ball ran loose to Essien who found no power or accuracy in his drive and the chance went begging.

Drogba then wasted a great chance to put Chelsea 2-1 in front on the night when he seized on the loose ball on the edge of the penalty area.

But the player who has smashed 30 goals for the Blues his season saw his effort saved by Canizares who dived low to his right to keep the Ivorian off the score sheet.

Valencia substitute Miguel Angel Angulo wasted a good chance when he shot wide of the target as the Spanish side pressed for the goal that would give them victory.

The Spanish side had Canizares to thank for keeping them in the game in the 84th minute when he produced a sensational one-handed save to keep out a header from Ballack.

But Essien put Chelsea into the semi-finals with a glorious angled drive in the 91st minute.

Manager Mourinho's joy was clearly evident as he jumped on defender John Terry and cheered as the England skipper tried to speak to a TV reporter after the match.

"I think he's happy now!" Terry said. "He wasn't at half-time.

"We had to be a lot better than in the first half. We were not good enough and we got a kick up the backside.

"We came out in the second half and we dominated. We never gave up and we had said before if we went a goal down it wouldn't change anything."

Mourinho said: "It was a great performance, especially in the second half. A great performance against a good team. We had a difficult start and were losing 1-0 and out of the competition.

"But the second half was amazing. We dominated the second half but I did not have to give a hot half-time talk. I was just telling my players to be strong and to have a strong mentality.

"I told them to resist the pressure. We made a tactical change by putting Essien at right-back. He is offensive and powerful in that position and Joe Cole and Shevchenko helped us to create more two-on-one situations.

"I am proud of this group because this is a special season. Nobody has had so many problems as we had. We have had big problems with more than 50% of the team.

"But they resist and resist and mentally they are very strong. I could not be more proud of these players.

He added: "Being in the semi-final is great for the team, great for the group.

"This is amazing. We have won a competition and are in two semi-finals.

"We have a difficult semi-final on Sunday but hopefully we can be in the FA Cup final and we are still fighting for the Premiership.

"It is an amazing situation for English football with three teams in the semi-finals of the competition.

"But we deserved to win the game over 90 minutes. [Santiago] Canizares made two unbelievable saves - especially the one against Michael Ballack's header.

"I thought the team was very strong. I had the changes ready if it went to extra-time. But when I saw the ball in the net in the last minute with Valencia needing to score two, it was great.

"It happened to me when I was at Porto - we scored in Old Trafford in the last moment. Like all goals, there was great emotion.

"Essien is a special player. He is an athlete. He can run forever. He could play a game every day. He is a superb athlete.

"When we lost him one month ago it was a big, big loss. But he was ready to come for this crucial game. I know he is suspended for the next game but this one was the crucial one. The next one is not crucial."

But Mourinho refused to be drawn on gaining revenge on Liverpool for their semi-final defeat of two years ago.

Mourinho added: "Revenge is not for me. It is not a word for me in football or life. We know we start from behind and why - it is simple because Liverpool plays only Champions League we play everything else.

"We have players suspended and on yellow cards and Liverpool is in a very clean situation so I think they start in front of us. But the mentality and the character my players are showing - we will be ready for them."

Shevchenko, often labelled a misfit for the way he has failed to live up to his billing during his inaugural season at Stamford Bridge, equalised with his 59th European goal seven minutes after the re-start.

Chelsea dominated the second half but extra-time loomed when Canizares produced a one-handed save to deny Ballack in the closing stages.

But Essien, playing his first game since hurting knee ligaments against Tottenham in the FA Cup last month, delivered a fabulous finish in the 90th minute when his angled drive beat Canizares at his near post.

Mourinho also insisted that he wants to remain at Chelsea next season but said ominously: "What I want is to remain in England with Chelsea but sometimes in life you cannot get what you wish and what you want.

"If I can't coach there, I would like to coach another team."