The big news from Camp Nou was that Barcelona skipper Carles Puyol started on the bench, with Marquez at centerback and Eric Abidal on the left. Visitors Chelsea, meanwhile, brought Alex back into their rearguard, with Didier Drogba ploughing a lone furrow up front. As expected, Jose Bosingwa was switched to the right with a view to thwarting Lionel Messi.
A sea of color and a wall of noise awaited the sides inside the stadium, and the hotly-anticipated semi-final first-leg was soon underway in Catalunya.
Just over a minute in, Barcelona's threat was glaringly evident. Thierry Henry won a free kick from Branislav Ivanovic on the left, from which Xavi swung the ball to Samuel Eto'o at the back post. The striker headed back across goal and Rafael Marquez went close at the other side, although the play had already been stopped for an infringement.
Chelsea did try to respond instantly, with Frank Lampard going wide with a passing shot after some neat play. However, the hosts very quickly settled into a rhythm.
For the first 20 minutes their movement and build-up play was fluid, but their decisions in the final third left much to be desired. After nine minutes, Messi tried to run into a crowded box when he should have shot.
Eto'o was always lurking around, too. Petr Cech spilled his cross and was lucky to get away with it; Messi couldn't quite find him at the back post; he sliced a half-volley wide. Still, this was second gear fare.
Midway through the opening period, the Blaugrana began to turn the screw. First, Messi attempted to charge into the box and went down after being clipped by Alex, who earned the game's first booking following a mixed bag of previous challenges.
Xavi then drove wide from the edge of the box as Barcelona's shooting from range began to improve. Soon, Henry blasted a couple of efforts into the arms of Cech, but the goalie was up to the task despite his recent inconsistent form.
From the half-hour mark onwards, the game started to heat up and tempers began to simmer. Bookings for Michael Ballack and Toure Yaya, plus a developing feud between Dani Alves and Blues' duo Florent Malouda and Drogba, actually seemed to serve the English side better.
Suddenly complacent, Marquez played a suicide pass straight to Drogba, and the Ivorian ambled through on goal. Victor Valdes made a strong stop from the initial shot, before superbly clawing the ball out of the air as the forward tried to chip the rebound over him.
Minutes later, Gerard Pique also panicked with Drogba lurking, but the striker slipped before he could pull the trigger and the chance was gone.
Despite a frustrating end to the half, Henry had one more chance to glide in from the left and pick out a pass, but he dithered and allowed Alex to close him down.
The second period began on a sour note: Marquez crumpled to the ground, holding his left knee, and had to be replaced by Puyol.
Around the hour-mark, Barcelona started to apply the pressure again. Pique tried to find Dani Alves with a quarterback pass, but Cech was quick to smother the danger.
Then Xavi arrowed a corner into the path of the coiled Messi, who smashed a left-footed volley over the bar.
Abidal tried to slide the ball into Messi from the left, but John Terry was there to make a good block.
Dani Alves then curled a free kick onto the top of the net after Frank Lampard had brought down Iniesta 20-yards out.
Now approaching the last 20 minutes, the hosts kept coming in waves. Eto'o surged forward on the break, cut inside the backtracking Alex and, as he shaped to shoot, Cech again did well to save with perfect timing.
The Cameroonian forward was soon in possession again inside the box, this time turning provider with Henry better placed. The Frenchman swiveled, but lost his footing, with some claiming a tug by Bosingwa.
On 74 minutes, substitute Puyol managed to rule himself out of the second leg, earning a yellow card for a poor challenge on Malouda. Coach Pep Guardiola could thus have some real selection headaches for the next leg.
The pressure continued to mount, but Chelsea stood firm – although Ballack was lucky to be spared a second yellow as he impeded Iniesta – and Barca never really had the murderous glint in their eyes. This was exemplified by sub Bojan Krkic right on the 90, who really should have kept his header down from point-blank range following Dani Alves' cross.
There was still time for another replacement, Aliaksandr Hleb, to spurn a golden opportunity however. His fresh legs beat the offside trap, but Cech narrowed the angle expertly and repelled the Belarusian.
Thus it finished, and Chelsea will perhaps feel better about the return leg having kept the first clean sheet at Camp Nou this term.
Greg Ptolomey, Goal.com