Favourites Barcelona recovered their poise to grab a valuable 1-1 draw at Olympique Lyon on Tuesday but holders Manchester United were frustrated on their Champions League return in a 0-0 draw at Inter Milan.
Barcelona, whose commanding form has deserted them of late, fell behind to a free kick from Juninho and needed a Thierry Henry header in the second half to take a slight edge from the first leg of their last-16 tie thanks to the away goal.
Cristiano Ronaldo went close for United early and late at the San Siro but Alex Ferguson, once again unable to get the better of Jose Mourinho, had to settle for extending their unbeaten run in the Champions League to a record 20 matches.
In the night's other Anglo-Italian clash, Arsenal clinched a 1-0 win at home to AS Roma thanks to a Robin van Persie penalty, making them the night's only winning team.
The other tie was left well poised, as Porto twice came from behind to snatch a 2-2 draw away to Atletico Madrid.
United made a furious start at the San Siro.
Ronaldo forced a great point-blank save from Julio Cesar from a corner and the chances came thick and fast in the first half. Inter improved after the break but nearly succumbed at the end, when again the keeper did wonderfully well to keep out a free kick from Ronaldo.
Barcelona have seen their lead in the Primera Liga cut from 12 points to seven in the last two games and they looked shaky at the back again as set-piece specialist Juninho deceived keeper Victor Valdes with a dipping free kick.
Barca looked clueless in the first half but they were much better in the second and got the deserved equaliser through Henry's low header at the far post.
Arsenal should really have beaten Roma by a clearer margin.
They got the only goal when Van Persie was fouled by Philippe Mexes after 37 minutes and got up to smash a low spot kick past keeper Doni.
The remaining four first leg matches in the first knockout round are being played on Wednesday, with nine-times champions Real Madrid at home to Liverpool in the outstanding tie.
Chelsea are at home to Juventus, Villarreal against Panathinaikos and Sporting against Bayern Munich.