Steaua Bucharest will take a lead to Stamford Bridge after Raul Rusescu's first-half penalty gave the Romanian side a slender 1-0 victory over Chelsea in the Europa League.
Ryan Bertrand's clumsy challenge on Rusescu gave the referee no choice but to point to the spot - and Rusescu drilled the spot-kick under Petr Cech.
Chelsea improved after the second half introduction of Juan Mata, with Eden Hazard and Marko Marin each going close late on.
But Steaua held on for victory, and Chelsea will have to reverse the deficit next Thursday to keep their Europa League hopes alive.
With one eye on Sunday's FA Cup quarter-final with Manchester United, Chelsea manager Rafael Benitez kept Ashley Cole, Cesar Azpilicueta, Gary Cahill and Juan Mata on the bench, with Demba Ba ineligible.
Chelsea spent the early stages on the back foot, soaking up Steaua's early pressure. The lively Rusescu was proving a handful from the outset, with Alexandru Chipciu also causing problems.
But Chelsea were defending very deep and invited the pressure and, while Steaua's first real chance, was the penalty - it was an opportunity the Romanian side had hard earned.
Trying to reach Iasmin Latovlevici's excellent cross from the left, Rusescu was impeded by Bertrand, whose ill-advised challenge saw the striker hauled to the ground - with the referee always likely to award the spot-kick. Rusescu dusted himself down and fired the penalty into the bottom left corner, under Cech's body.
Chelsea responded with their best chance of the game when Hazard and Yossi Benayoun exchanged passes on the edge of the box - but the Israeli's low shot lacked conviction and was palmed around the post well by Ciprian Tatarusanu.
Chelsea improved after the break but Steaua, backed by a sell-out home crowd, remained comfortable against a Chelsea side that were sloppy in the final third, with Torres isolated for long periods.
Benitez threw on Juan Mata as he looked to unlock the Steaua defence, and the Spaniard had an immediate impact. But Chelsea's best chances feel to Hazard, who lashed Frank Lampard's deep cross over the bar from a tight angle, and Marin, who twisted his way into a shooting position in the box - but Steaua goalkeeper Tatarusanu pulled off another good save low to his right.
Steaua held on for a deserved win and, perhaps crucially, prevented Chelsea scoring an away goal. The sides meet again for the second leg next week.
MAN OF THE MATCH
Ciprian Tatarusanu (Steaua Bucharest) - The Steaua goalkeeper was in inspired form, pulling off excellent saves from Yossi Benayoun in the first half and Marko Marin late on.
STEAUA BUCHAREST: Tatarusanu 8, Rapa 7, Szukala 7, Chiriches 7, Tatarusanu 8, Bourceanu 7, Pintili 6, Popa 6, Chipciu 7, Tanase 7, Rusescu 8 Subs: Prepelita 6, Gardos N/A, Tatu N/A.
CHELSEA: Cech 7, Ivanovic 6, Terry 6, Luiz 7, Bertrand 6, Mikel 6, Lampard 6, Benayoun 5, Hazard 7, Oscar 6, Torres 5. Subs: Mata 7, Marin 7