Berlin - Ghana internationals Kevin-Prince Boateng and Sulley Muntari hit second-half goals as AC Milan stunned Barcelona 2-0, while Schalke shrugged off a form crisis with a 1-1 draw at Galatasaray in first-leg matches of the last 16 of the Champions League on Wednesday.
Barcelona, seeking a fourth European title since 2006, were unable to break down seven-time champions Milan and were twice punished at the back to suffer their second defeat of the current competition.
"It's a really bad result," Barcelona defender Gerard Pique said.
"We played badly and can't make any excuses. When they scored the first we lost control. This is a difficult place to play and Milan are a club with a big history.
"We will obviously try to win the return game. We must improve and learn from tonight. We will look to respond on one of those magical nights at the Camp Nou."
Milan kept Lionel Messi quiet and squeezed out Barcelona's midfield, with the Spanish league leaders - who had been beaten by Celtic in the group stage in Glasgow - unable to carve out a real chance until a late Carles Puyol header wide of the post.
Milan went closest in a first half of few chances when Boateng shaved the post with a shot across goal.
And it was Boateng who broke the deadlock in the 57th minute, although there was a suspicion of handball by team-mate Cristian Zapata.
Zapata had his hands raised on the edge of the area when he was struck -on the head, as television replays showed - by Riccardo Montolivo's 30-metre shot from a free-kick.
Boateng, however, did not hesitate when the ball fell to him inside the area, and he swept the ball home with a left-foot shot.
Milan hit Barcelona again with nine minutes remaining when substitute M'Baye Niang won a challenge with Puyol before Stephan El Shaarawy found Muntari who finished with a left-foot shot.
"We listened to our coach (Massimiliano Allegri), because he has been fantastic in the way he set us up. We were very disciplined on the pitch," Muntari said.
"Last week I was telling my wife I've played in the Champions League and never scored so I was very surprised and very happy with the goal and the win for the team."
In Istanbul, Galatasary, fielding winter signings Didier Drogba and Wesley Sneijder, had to settle for a draw with Schalke, who now have the edge for the return in Gelsenkirchen.
Burak Yilmaz scored his seventh Champions League goal in seven games to put Galatasaray ahead in the 12th minute, but Jermaine Jones levelled for Schalke just before half-time.
Burak flicked the ball with his heel over captain Benedikt Hoewedes before smashing the ball into the roof of the net for a brilliant open for the Turkish league leaders.
Schalke almost levelled when Hoewedes headed on a corner which Klaas-Jan Huntelaar at the far post just needed to nudge over the line, but the Dutch striker failed to connect.
Former Chelsea striker Drogba, back in Champions League action following his winter signing for Galatasaray, was involved in the next big chance, with his shot saved by Timo Hildebrand before Hamit Altintop followed up to hit the bar.
Hildebrand then saved well from Burak, but it was Schalke who brought an end-to-end first-half to an end with a clinical counter attack finished off by Jones from a pass by Jefferson Farfan.
"The first half was good and it was a pity we conceded a goal through a mistake but it was a good performance," Huntelaar said.
Jones, who will be suspended for the return leg after a booking, said: "I think we can be very satisfied. We knew it would be a hard game but on the whole it was a good performance.
"I think we saw a different face from Schalke to what we have been showing in the Bundesliga."
The games complete the first-leg ties of the last 16.
On Tuesday, Bayern Munich were 3-1 winners at Arsenal, while Porto defeated Malaga 1-0.
In last week's games, Real Madrid and Manchester United finished 1-1, Shakhtar Donetsk and Borussia Dortmund drew 2-2, Juventus were 3-0 winners at Celtic, and Paris St Germain earned a 2-1 victory at Valencia.
The return legs are on March 5 and 6, and March 12 and 13.