Leo Messi's hat trick powered FC Barcelona to a 4-0 romp over Ajax, while a heavily favored Chelsea side lost 2-1 at home to Basel in Wednesday's Champions League action.
The day's other upset came in Italy, where visiting Borussia Dortmund - last year's losing finalist - was defeated 2-1 by Napoli.
Atletico Madrid beat Zenit St. Petersburg 3-1 in the Spanish capital. Schalke 04, meanwhile, erupted for three late goals in a 3-0 home win over Steaua Bucharest.
Visiting Celtic managed to hold AC Milan scoreless until the 82nd minute, but still lost 2-0.
Two clubs, Arsenal and Porto, chalked up away victories on Wednesday. The Gunners topped Olympique Marseille 2-1, while the Portuguese side shut out Austria Wien 1-0.
Basel, on nobody's list of Champions League contenders, dealt Chelsea an embarrassing loss in front of the Blues faithful at Stamford Bridge.
Dortmund's defeat was marked by some sideline histrionics from the German squad's coach Jurgen Klopp, whose protest over an official's call got him ejected. EFE