Kevin Behrens scored a hat-trick on Sunday to guide Union Berlin to a 4-1 Bundesliga season-opening win over Mainz 05, who had two penalties saved.

The hosts needed only 52 seconds to open their account with Behrens' glancing header from a smooth Jerome Roussillon cross becoming Union's quickest ever Bundesliga goal.

The Berliners, playing in the Champions League group stage this season after their stunning fourth-place finish, doubled their lead when the 32-year-old Behrens was left with too much space in the box and scored with a towering header from Aissa Laidouni's cross.

Union hit the crossbar in the 32nd minute before Mainz keeper Robin Zentner pulled off the save of the match, tipping a curled shot by Union's Diogo Leite over the crossbar in first- half stoppage time.

Mainz came out fighting after the break and earned a 63rd- minute penalty but striker Ludovic Ajorque's weak effort was easily saved by Frederik Ronnow.

The keeper, however, was beaten a minute later by Anthony Caci's thunderous volley into the top corner that injected new life into Mainz.

Their joy was shortlived, however, with Behrens completing his hat-trick with another powerful header.

Frenchman Ajorque hit the crossbar before Union earned a second spot-kick in the 87th minute for handball.

Ajorque stepped up again, only to see Ronnow block his second spot-kick as well and Union wrapped up the three points with a stoppage-time goal from Milos Pantovic.