Their manager, Michael Laudrup, brought in Sennels to fill the left-back position previously held by Aurelijus Skarbalius.

[7] However, he gradually found his rhythm and went on to appear in all 33 matches for Brøndby in the 2004–05 season, during which the club secured both the league title and the Danish Cup.

[4][9][10] When René Meulensteen took over as manager of Brøndby, Sennels faced even tougher competition for his position due to the arrival of on-loan left-back Adam Eckersley from Manchester United.

[11] With Meulensteen's departure halfway through the 2006–07 season, Sennels was informed by the caretaker manager, Tom Køhlert, that he had no future at the club.

[18][17][19] He made his Denmark international debut in a friendly against Finland at Parken on 20 August, replacing René Henriksen after 64 minutes of a 1–1 draw.