Lionel Messi preserved Barcelona's unbeaten start to the Champions League by getting the tying goal in a 1-1 draw at AC Milan while Arsenal's perfect record ended with a 2-1 loss at home to Borussia Dortmund on Tuesday.
Atletico Madrid made it three wins in three matches thanks to two goals from striker Diego Costa in a 3-0 victory at Austria Vienna.
Chelsea claimed the performance of the night by beating Schalke 3-0, with striker Fernando Torres also scoring twice to put the 2012 champions on top of Group E.
Back from a thigh injury, Messi scored from close range to match Robinho's opener for Milan and keep Barca in charge of Group H on seven points, two ahead of the Italians.
Robert Lewandowski grabbed an 82nd-minute winner for Dortmund, last season's finalist, at Arsenal in Group F. They are joined in a three-way tie with six points by Napoli, which won 2-1 at Marseille to send the French side to a third straight loss.
Celtic and Zenit St. Petersburg got their first wins while Basel drew 1-1 at Steaua Bucharest.
Milan and Barca were meeting in Europe's top competition for the seventh time in three seasons.
There was no shortage of stars at the San Siro and two of them combined for Milan's opener, with Robinho exchanging passes with Kaka before slotting past Victor Valdes.
However, Milan has never been able to handle Messi — the Argentina forward has now scored six times against the Italians — and he equalized after collecting Andres Iniesta's pass, evading Kevin Constant and shooting into the bottom corner.
The teams meet again in two weeks, with Barcelona knowing a victory would give them 10 points and close to the knockout stage.
Celtic is two points behind Milan after beating Ajax 2-1 thanks to goals by James Forrest and Beram Kayal in a 10-minute spell either side of halftime.
Outpatient ROBOTIC HYSTERECTOMY. Trust an experienced Robotic Surgeon.