Porto became the fourth team to score four goals on matchday one as they fought back to beat København while Monaco's Irvin Cardona registered the first hat-trick of the 2016/17 competition.
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Bayer Leverkusen 2-1 CSKA Moskva (Rhein 28, Akkaynak 39; Pukhov 35pen)
Midfielder Atakan Akkaynak, one of UEFA.com's ten to watch from the 2016 UEFA European Under-17 Championship in Azerbaijan, struck what proved the winner six minutes before half-time.
Tottenham Hotspur 2-3 Monaco (Shashoua 24, Harrison 78pen; Cardona 7 43 62)
Irvin Cardona's hat-trick, the first of this fledgling UEFA Youth League season, was decisive for the visitors.
Real Madrid 1-1 Sporting CP (Diaz Velazquez 55; Rosendo Marques 90+3)
Rosendo Marques's last-gasp effort denied last season's semi-finalists a winning start.
Juventus 2-1 Sevilla (Kean 35, Bove 90+6; Mena 53)
The game was deep into added time when Gabriele Bove beat Sevilla keeper Juan Soriano with a free-kick.