Cristiano Ronaldo missed an early penalty as Manchester United ground out a 0-0 draw against Barcelona in the first leg of their Champions League semi-final at the Nou Camp.

The visitors could have gone ahead in the third minute when they were awarded a penalty for Gabriel Milito's handball, but Ronaldo placed a poor penalty wide of the post.

United were forced to defend for large portions of the game, but a disciplined defensive effort reduced their hosts to pot-shots from distance that rarely troubled goalkeeper Edwin van der Sar.

And despite the missed opportunity of Ronaldo's penalty, the result probably makes United slight favourites ahead of the second leg.

The Premier League leaders took to the field shorn of the services of centre-half Nemanja Vidic - laid low by a stomach bug - and Sir Alex Ferguson decided to replace him with Wes Brown and move Owen Hargreaves to right-back.

Ronaldo began the game behind Carlos Tevez in attack with Wayne Rooney starting on the right, while Lionel Messi made his first start in seven weeks for Barcelona after overcoming a thigh problem.

One of the most keenly contested pre-match talking points was the competing claims of Messi and Ronaldo for the title of best player in the world, and the United man was handed a golden chance to score a precious away goal when Milito used a hand to block his header and referee Massimo Busacca pointed to the spot.

But United's number seven, usually so reliable from 12 yards, placed his spot-kick wide of the right-hand upright, and the shock of nearly going behind so early in the tie stunned Barca into action.

First Andres Iniesta just failed to pick out a team-mate after being released by a cute ball down the inside-left channel from Messi, and then Messi seized on a scuffed clearance from Van der Sar to feed Samuel Eto'o inside the United area, only for Eto'o's attempted return pass to be intercepted.

Despite enjoying the lion's share of possession, Barca struggled to craft any clear-cut openings, and after half-an-hour Ronaldo had a second strong penalty appeal turned down when he was bundled over by Rafael Marquez after latching onto Iniesta's loose pass.

The darting Messi was at the heart of all his side's best football, but all too often their attacks ran out of steam when confronted by the massed ranks of white shirts defending the threshold of the United penalty area.

Chances were more forthcoming in the opening stages of the second period, with Gianluca Zambrotta forcing Van der Sar to touch a 30-yard effort over the crossbar and Michael Carrick producing a timely intervention to cut out Eto'o's cross after the Barca striker had skipped past a lunging challenge from Rio Ferdinand.

Eto'o then slammed a low shot into the side netting after a neat touch from Iniesta, before a rising left-footed effort from Carrick met the same fate at the other end.

Ronaldo had another penalty shout waved away after being halted in his tracks by an airborne challenge from Eric Abidal, before Frank Rijkaard revealed his final hand with the introduction of Thierry Henry from the bench.

The Frenchman was so often the scourge of Ferguson's men in his Arsenal days and he stung Van der Sar's gloves with a well-struck effort from the left-hand corner of the area with just under 10 minutes remaining.

But it was the nearest Barca game to breaking the deadlock, meaning there is all to play for at Old Trafford next week.

Tom Williams / Eurosport