Bayer Leverkusen tightened their grip on second place in the Bundesliga as they cruised to a 2-0 victory at struggling Holstein Kiel on Saturday while Serhou Guirassy hit four goals as "euphoric" Borussia Dortmund routed Union Berlin 6-0.

The victory allowed German champions Leverkusen to cut Bayern Munich's advantage to five points before the leaders host third-place Frankfurt on Sunday.

Bayern had ground out a 0-0 draw at Leverkusen in the previous round and the teams have been drawn to meet in the Champions League round of 16, with the first leg on March 5.

Leverkusen dominated at last-place Kiel. The visitors took the lead after 10 minutes. Home goalkeeper Timon Weiner pushed a cross from Amine Adli to Patrik Schick, who tapped home.

Adli added a second on the stroke of half time, collecting a pass from Florian Wirtz and chipping Weiner.

Leverkusen have now gone 28 away games without defeat, a record for the club.

However, it was a statistic which did not excite coach Xavi Alonso.

"It's not the most important thing and we don't get a medal for it," said Alonso.

"But it means that we are very stable and that we have a top mentality. We don't want to stop now, we want to keep going."

Mainz moved up to fifth with a 2-0 home victory over St Pauli.

Lee Jae-sung poked home a rebound to give the hosts a 67th minute lead. Paul Nebel hit the second on a breakaway deep into added time.

In Moenchenglabach, home goalkeeper Jonas Omlin was sent off after 28 minutes for bringing down Phillip Tietz who was running in unchallenged at goal.

French striker Alexis Claude-Maurice made Borussia pay with a 15-minute second-half hat-trick as Augsburg won 3-0.

Dortmund trounced Union Berlin 6-0 with four goals from Serhou Guirassy which took his season tally in the Bundesliga to 13.

Dortmund, who are still involved in the Champions League and face Lille in the last 16 next month, are 10th in the table with 32 points.

"Everyone is relieved, we haven't seen the spectators so euphoric in a long time," Dortmund sporting director Sebastian Kehl told Sky.

"Serhou put on a huge show.

An own goal from Diogo Leite gave Dortmund a 25th-minute lead with Maximilian Beier adding the sixth in the closing moments.

Bochum crept out of the bottom two with a 1-1 draw at Wolfsburg, where both goals came from headers after corners.

Erhan Masovic gave the visitors a 50th minute lead. Mattias Svanberg forced home an 81st minute leveller.