Galatasaray, Fenerbahce, Beşiktaş and Adana Demir all competed in European competition qualifiers this week and all won.
Galatasaray beat Zalgiris 1-0 to progress to the next round of UEFA Champions League qualifiers.
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Mertens scored a cracking long-range effort to win the game and take the Lions through to the next round where there will face Maribor.
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Fenerbahce meanwhile beat 4-0 to win 9-0 on aggregate to progress to the next round.
Dusan Tadic, Michy Batshuayi scored a brace and Ismail Yuksek all got their names on the scoresheet.
The Yellow Canaries will face Maribor next in the Europa Conference League Third qualifying round.
Besiktas meanwhile beat Tirana 2-0 in Albania in the Conference League.
Daniel Amartey scored on his debut after joining from Leicester City and Vincent Aboubakar got his name on the scoresheet to wrap up a 5-1 victory on aggregate.
The Black Eagles will face Neftci next in the competition.
Adana Demirspor had the toughest fixture playing Romanian outfit Club.
The Super Lig side ended up 3-2 on aggregate but it was a hard won contest.
David Babajide opened the scoring for the home side but things took a turn for the worse when Andreaw Gravillon got sent off on 63 minutes.
And things got even worse when Vasile Mogos equalised from the penalty spot.
The home side got the last laugh with Cherif Ndiaye scoring a last gasp winner in the third minute of additional team.
Adana face Osijek in the next round.