UEFA Champions League defending champions Liverpool suffered a 2-0 defeat away to Napoli in their opening match.
Dries Mertens and Fernando Llorente scored a goal each in the victory.
Meanwhile Chelsea lost 1-0 to Spanish side Valencia.
Rodrigo scored for the hosts in the second half.
In other matches Barcelona and Borussia Dortmund drew nil all, while Ajax beat Lille 3-0.
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