David Silva's blistering drive gave Valencia a crucial away goal but Didier Drogba kept Chelsea's hopes alive in this Champions League quarter-final.
Drogba scored his 30th goal of the season for Chelsea
Silva capped an outstanding display with a fierce shot into the top corner to open the scoring on 30 minutes.
Earlier, Salomon Kalou cracked an effort onto the bar after Andrei Shevchenko had been poleaxed by Roberto Ayala yet denied a penalty.
On 53 minutes, Drogba nodded home to leave the second leg finely balanced.
However, Chelsea may feel they could have got more out of the game after coming back from a shaky opening to dominate the second half.
And they may also reflect that they could well have been playing against 10 men had the referee taken action against Ayala for smashing into Shevchenko's face with his shoulder with just nine minutes played.
Whether the challenge was just outside the box or not is debatable - that the referee did not even deem it a foul was bemusing.
Valencia's good luck continued as Kalou's sweetly-struck shot pinged back off the frame of the goal.
But fortune supposedly favours the brave and Valencia - in an attacking 4-4-2 line-up despite missing seven first-team players - soon showed they were just that.
After overcoming initial pressure from Chelsea and coming to terms with the darting runs of Kalou and a more authoritative performance from Michael Ballack, Valencia began to settle.
Silva, dropping deep behind his strike partner David Villa, started to dictate the tempo as well as leave Jon Obi Mikel chasing shadows.
Mikel, preferred to Claude Makelele, got away with loose marking when Silva - completely free in the box - could not quite latch on to Joaquin's dragged shot.
But the Blues midfielder was severely punished when Silva again found space, cut in from the left flank and cracked an audacious effort beyond Petr Cech, who was left groping the air as the ball powered past.
A laboured-looking Chelsea desperately needed to play with more pace and purpose but it took them until after the interval to do so.
The first chance of the half came quickly as Lassana Diarra picked out Kalou with a pin-point centre, but the striker swung at the ball and missed.
However, soon after Drogba showed how to finish clinically when his pace got him to Ashley Cole's hopeful punt forward and he headed the ball over Santiago Canizares.
It was Drogba's 30th goal of the season - the first time a Chelsea player has hit that mark since Kerry Dixon.
Yet, if the Chelsea faithful were expecting Drogba's strike to set up victory, that was not to be the case.
Despite plenty of possession - and Mikel's improvement at shackling Silva - Chelsea could not fashion any clear-cut chances.
Ricardo Carvalho's hesitant prodded shot was typical - and Shevchenko's superb overheard kick which followed it was deflected over.
And even the return of Joe Cole could not inspire Chelsea, but they will need to be more creative and potent in the return leg in Spain, as they know they must score to survive.
Chelsea: Cech, Diarra, Terry, Carvalho, Ashley Cole, Kalou (Wright-Phillips 74), Ballack, Lampard, Mikel (Joe Cole 74), Shevchenko, Drogba.
Subs Not Used: Cudicini, Makelele, Boulahrouz, Bridge, Ferreira.
Booked: Drogba, Diarra.
Goals: Drogba 53.
Valencia: Canizares, Miguel, Ayala, Moretti, Del Horno, Vicente (Angulo 57), Albelda, Albiol, Joaquin (Hugo Viana 86), Silva, Villa (Jorge Lopez 90).
Subs Not Used: Butelle, Curro Torres, Nacho Insa, Pallardo. Booked: Silva, Albelda, Ayala.
Goals: Silva 30.
Ref: Frank De Bleeckere (Belgium).