Anderlecht, like Ajax once a major force in continental competitions, snatched a late 1-1 draw at home to Manchester United in another of the quarter-final first-leg ties while Celta Vigo beat visitors Genk 3-2 in a goalfest.
There was an untimely reminder that the hooliganism which once plagued European football has still not been completely eradicated when crowd trouble delayed the start of Olympique Lyonnais' match at home to Turkish side Besiktas by 45 minutes.
Dozens of fans spilled onto the pitch, with the French club saying the supporters were seeking refuge after being pelted with objects and fireworks thrown from high up in the stands.
Ryan Babel gave Besiktas the lead after 15 minutes but the hosts, who eliminated AS Roma in the previous round, replied twice in the last 10 minutes for a dramatic 2-1 win.
Corentin Tolisso equalised in the 83rd minute after Mathieu Valbuena's free kick deflected invitingly into his path and Jeremy Morel caught Besiktas goalkeeper Fabricio in possession two minutes later before scoring into an empty net.
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Fans invade the pitch during Olympique Lyonnais v Besiktas match Reuters
Four-times European champions Ajax, now effectively reduced to the role of a springboard for young players hoping to move to the bigger leagues, enjoyed a reminder of what continental success was like as they outclassed Bundesliga side Schalke.
Ajax played plenty of the flowing football for which their predecessors were renowned and went ahead when Davy Klaassen converted a penalty after Amin Younes went down under a tackle from Alessandro Schoepf.
Klaassen struck again early in the second half, firing home from 12 metres after he was picked out by Justin Kluivert, the son of former Dutch international Patrick.
Henrikh Mkhitaryan gave Manchester United a 37th minute lead in Belgium after snapping up a rebound, and the visitors seemed on the way to a comfortable win as they dominated the game.
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Manchester United's Henrikh Mkhitaryan scores their first goal Reuters
Leander Dendoncker had other ideas as he outjumped Matteo Darmian to head in Ivan Obradovic's cross with four minutes left.
Jean-Paul Boetius headed Genk into a 10th minute lead in Spain but Celta hit back quickly as Pione Sisto, Iago Aspas and John Guidetti made it 3-1 before halftime.
Thomas Buffel pulled a goal back midway through the second period to revive Genk's hopes and leave the tie on a knife-edge.
Olympique Lyon 2
Corentin Tolisso 83, Jeremy Morel 84 Besiktas 1
Ryan Babel 15
Leander Dendoncker 86 Manchester United 1
Henrikh Mkhitaryan 37
Celta Vigo 3
Pione Sisto 15, Iago Aspas 18, John Guidetti 38 Racing Genk 2
Jean-Paul Boetius 10, Thomas Buffel 67
Ajax Amsterdam 2
Davy Klaassen 23pen,52 Schalke 04 0