Brighton recorded 50 touches inside the Tromso box, won 11 corners and completed 89% of their passes during the first leg, yet still returned from Norway with a 0-0 draw. Fabian Hurzeler's side now need any victory at the Amex on Thursday, with the second leg kicking off at 7.30pm.
The xG numbers were closer than Brighton's territorial control suggested, with the visitors generating 1.65 compared with Tromso's 1.36. Jakob Haugaard made two excellent saves during the first half, diving low to deny Georginio Rutter before keeping out Ibrahim Osman's volley.
Tromso carried a genuine counterattacking threat despite spending long periods without the ball. Heine Larsen beat Brighton's offside trap during the second half but saw Bart Verbruggen block his one-on-one attempt, while Isak Vadebu broke through in the 85th minute before shooting wide.
Four big chances were missed across the match, showing why the first leg remained goalless despite both sides creating enough to score. Brighton controlled the territory, but Tromso produced the two clearest counterattacking opportunities after escaping the home defence.
Club-record signing Luka Vuskovic made his competitive debut in central defence and had a relatively quiet evening apart from a second-half booking. Fellow summer arrival Zadok Yohanna was introduced from the bench late on.
Brighton's response in the Premier League was emphatic, beating Europa League winners Aston Villa 4-0 at the Amex. That victory was supported by an xG advantage of 3.19 to 0.33, giving the result considerably more underlying weight than the scoreless first leg in Norway.
Maxim De Cuyper forced Victor Lindelof into an own goal inside eight minutes before scoring the second himself 10 minutes later. Jack Hinshelwood then added two goals in quick succession around the half-hour mark to put the match beyond Villa.
Joao Gomes was sent off for kicking out at Rutter during a difficult full debut, although Brighton were already in control before Villa were reduced to 10 players. Vuskovic started again and looked more assured in his first Premier League appearance.
Tromso prepared for the return leg with a 3-2 defeat away at Molde, although their wider domestic record remains strong. They sit third in the Eliteserien after 17 matches, recording 10 wins, four draws and three defeats.
Their away record stands at four wins, two draws and two defeats from eight games, with a +3 goal difference. Only two Eliteserien sides have collected more away points, confirming Tromso are stronger than their status as heavy outsiders suggests.
How the bookies view it
Brighton are 1/6 to win, implying an 85.7% chance. The draw is priced at 15/2 and carries an implied probability of 11.8%, while Tromso are 16/1 at 5.9%.
Over 2.5 goals is available at 19/50, implying a 72.5% chance. Both teams to score is priced at 11/10 and carries an implied probability of 47.6%.
Head to head: Only second meeting
The first leg was the only previous meeting between Brighton and Tromso, finishing goalless despite both teams creating clear chances.
Player to watch: Georginio Rutter looking to cement main striking role for hosts
Georginio Rutter made 32 Premier League appearances last season, starting 20 and contributing three goals and three assists. He recorded 42 shots but placed only 14 on target, showing his value came as much from movement and link-up play as finishing.
Rutter is now operating as Brighton's main striker following Danny Welbeck's departure. He did not score against Villa but remained central to their attacking play, including the cutback that created De Cuyper's goal.
He also produced Brighton's best attempt during the first leg, forcing Haugaard into a strong diving save with a low strike. Breaking down Tromso's deep defensive shape should again depend heavily on Rutter's movement between the lines.
Predicted line-ups
Brighton (4-2-3-1): Verbruggen; Costinha, Vuskovic, Boscagli, De Cuyper; Yalcouye, Ayari; Gomez, Hinshelwood, Osman; Rutter.
Tromso (5-3-2): Haugaard; Dahlqvist, Vadebu, Skjaervik, Tonnessen, Warneryd; Edvardsson, Jenssen, Larsen; Nyhammer, Larsen.
Anything else catch the eye?
Brighton's territorial dominance provides the clearest indication of how the second leg should develop. They had 50 touches inside the Tromso penalty area compared with only eight for their opponents, while winning the corner count 11-5.
Their 89% passing accuracy was also well ahead of Tromso's 72%, showing the difference in control whenever Brighton established possession. Tromso responded by making 32 clearances compared with Brighton's six and winning 18 aerial duels to 13.
That defensive approach was based on blocking space and clearing crosses rather than playing through Brighton's pressure. It worked in Norway, but sustaining the same resistance at the Amex should be more difficult after Brighton's four-goal performance against Villa.
Tromso still possess a counterattacking threat as they found space behind Brighton's defensive line during the first leg. Their away npxG ratio of 0.46 and negative shot balance suggest those opportunities are unlikely to arrive regularly.
Brighton remain without Yankuba Minteh, Evan Ferguson, Kaoru Mitoma and Stefanos Tzimas, while Tromso are missing Daniel Braut and Leo Cornic. Those absences reduce Brighton's attacking options but did not prevent them creating 3.19 xG against Villa.
The result market offers little interest at 1/6, while over 2.5 goals requires a more open game than the first leg produced. Brighton need only one goal to move ahead and have little reason to turn the tie into an end-to-end contest if they establish a lead.
The main bet is Brighton to win and under four goals. Their control of possession and territory should be enough to secure qualification, while Tromso's modest away attacking data and deep defensive setup point towards a controlled scoreline rather than a high-scoring contest.
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