Both Polish teams were convincing winners on the night, with last season's Ekstraklasa champions Lech Poznan scoring seven times in a 7-0 demolition of Thun.
Patrik Walemar's opener 10 minutes in gave Kolejorz the perfect start, but a three-goal haul in the space of 11 minutes later on in the first half practically ended the tie.
A fifth was added just before the break, with two more scored in the second half as Thun had no answers to their opponents' ruthless finishing.
Lech, who last reached the Europa League proper in 2020/21, also scored seven times last summer in a Champions League qualifier against Breidablik. However, Thursday's victory was their largest-ever in a UEFA club competition match.
The seven goals conceded are already the most Thun have let in across a two-legged tie.
Jagiellonia Bialystok also earned a convincing result, beating Iberia 1999 4-0 at home.
A pair of goals in either half did the trick for Jagiellonia as they near a maiden UEL group phase appearance. Remarkably, the Georgian side amassed 18 attempts in the contest, even having a goal ruled out for a foul.
Jaga are already guaranteed European football in the fall for a third year running after going as far as the Conference League quarter-finals back in 2024/25.
Earlier on in the day, Anderlecht defeated 10-man Kairat Almaty 3-0 to set themselves in a strong position to advance.
The damage was done in the first half by the Belgian side, who went up a man after Alexandr Shirobokov was shown a red card for a dangerous tackle.
Fellow Belgian side Sint-Truiden needed a stoppage time winner from Shogo Taniguchi to help them to a narrow 1-0 success against Omonia Nicosia.
The side from Limburg are 90 minutes away from becoming the 10th different Belgian club to feature in the Europa League group phase under the competition's current name.
Turkish duo Besiktas and Trabzonspor had mixed results on the day. The former had little problems with Kauno Zalgiris, winning 3-0 and all but book their spot in the league phase.
Trabzonspor had it more difficult, falling 1-0 to European regulars Ferencvaros. Kristoffer Zachariassen was the only goalscorer in Turkiye as the Hungarians claimed their second win in three head-to-heads (2-0-1).
Benfica get the job done
There were only two matches in the play-off round first legs that saw both teams score. One came in Lisbon, where Benfica battled past AGF Aarhus 3-1.
Rafa Silva needed less than two minutes to put the hosts ahead. Leandro Barreiro doubled their advantage at the half hour mark.
AGF, already assured a debut showing in a UEFA club competition group phase, got one back five minutes later, only to concede again as the half came to a close.
Benfica's last appearance in the Europa league's group stage came in 2020/21, when they finished second in their section before losing to Arsenal in the round of 32.
In Romania, Univeristatea Craiova and Ararat-Armenia played to a 1-1 draw in a match where the Romanians had the best of the chances. Yet, they had to play from behind, keeping them in the tie.
The other stalemate came in Albania, where Egnatia held Lillestrom to a goalless draw.
Former UEFA Cup finalists Crvena zvezda came away with a 3-0 victory at home to Viktoria Plzen. Mirko Ivanic scored twice for the Serbian champions, who claimed their fifth win in 10 against Czech-based opposition.
Greek Cup holders OFI Crete also prevailed 3-0 in their game, beating CSKA Sofia at home. It is their first win in continental play since 2000.
An own goal in stoppage time helped RB Salzburg to a 1-0 defeat of Mjallby in Sweden.
Salzburg's last victory on Swedish soil also happened to be in the Europa League, back when they won all six of their group games in the 2013/14 campaign.
All 12 return legs are next Thursday, with the league phase draw to occur the next day. The 12 losing sides will drop to the UEFA Conference League, which will also have their draw next Friday.
Follow all of the UEFA Europa League play-off round on Flashscore.