Current Champions League winners Liverpool, began the defence of their title on a bad note, losing 2-0 to Napoli.
With the game looking to end goalless, the hosts were awarded a penalty with eight minutes left.
Dries Mertens stepped up and gave his side the lead from the spot.
In added time, Fernando Llorente pounced on a loose pass, to seal the win.
Rodrigo scored the only goal, as Valencia beat Chelsea 1-0 at Stamford Bridge.
The game between Borussia Dortmund and Barcelona ended 0-0, with Marcos Reus missing a penalty in the second half.
Ajax thrashed Lille 3-0, Inter Milan drew 1-1 with Slavia Prague and RB Leipzig won 2-1 at Benfica.
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