Few would have expected St. Gallen to come out on top against the heavily-favoured Benfica, two-time Europa League finalists. Yet, the Swiss side found a way, prevailing 2-1 at home.
Aliou Balde put the hosts in front eight minutes before the break before Rafa Silva responded for Benfica, who lost only once in seven previous meetings against teams from Switzerland.
The Portuguese heavyweights responded with some more energy in the second half, carrying much of the play in search of a winner. Instead, it was Tom Gaal to grab the deciding goal for St. Gallen, who are now 90 minutes away from their biggest European scalp since the beginning of the millennium.
Benfica have never gone a season without playing in the group phase of the Champions League or the Europa League in the latter competition's current setup.
Thursday had started with a 0-0 draw in Azerbaijan between Qarabag and CSKA Sofia. Despite a total of 27 shots between the two sides - the home side edging it 14-13 - no one could find the back of the net.
That was one of two draws across the UEL's second qualifying round first legs. The other one came in Sweden, where Hammarby scored in the 89th minute through Frank Adjei to take a 1-1 draw with them to Belgium versus Anderlecht.
Adjei was only on the pitch for a matter of moments before his effort from the edge of the box flew into the back of the net.
The Belgian side, who lost to Hacken at this stage last term, led a few moments into the contest via a Danylo Sikan free kick, but were reduced to 10 men late in the second half after Nathan Saliba picked up his second yellow card.
Maccabi Tel Aviv hit five as PAOK take narrow lead
Israel's Maccabi Tel Aviv were the big winners for the day, beating Sheriff Tiraspol 5-0 on their travels. Sayd Abu Fahri scored in both halves for the away side, who have one firm foot in the third qualifying round.
The game marked the sixth time Maccabi scored five or more goals in a UEFA club competition match - they also scored five in a 5-1 beating of TSC two years ago in the playoff round.
The defeat was Sheriff's heaviest European home loss, marking the first time they conceded more than three goals at Sheriff Arena.
PAOK Thessaloniki were the other team involved in a five-goal game as they edged Dynamo Kyiv 3-2 in Lublin.
It was the hosts who went in front less than five minutes in, only for the Greek outfit to battle back and take the lead by half time. A third was added not long after the restart, but Dynamo got themselves a second towards the end to halve the deficit and keep them in the tie.
FC Midtjylland will be relieved they are still in their tie against Besiktas. Playing down a man since the 15th minute through Friday Etim red card, the Danes somehow managed to concede only once in their 1-0 reverse.
Besiktas, playing their first-ever European match against a Danish side, got their only goal in the 25th minute in a clash in which they had 28 attempts.
Elsewhere, Pafos fell 2-0 away to Hajduk Split. The goals came eight minutes apart in the second half as the Croatians close in on a potential third qualifying round tie.
In Enschede, Ferencvaros defeated Twente 2-1 courtesy of Lenny Joseph's brace. The Haitian forward scored what proved to be the winner with 10 minutes to go as the Hungarian Cup winners earned a key away victory.
Hradec Kralove won the other game on Thursday, beating Tromso of Norway 1-0.
Next Thursday will see the second qualifying round conclude, with the nine winning sides progressing to the penultimate round of UEL qualification. The defeated sides will transfer to the Conference League third qualifying round.
Follow all of the UEFA Europa League qualifying matches on Flashscore.