The Spaniard's side faced a tough test against Union at BayArena, with the visitor knowing a victory would see it reclaim top spot in the Bundesliga ahead of Bayern Munich.

After a goalless first half, Robert Andrich broke the deadlock a minute after the restart to spark a frantic second period, with a brace from Moussa Diaby followed by efforts from Adam Hlozek and Mitchel Bakker.

The win was the joint-highest win for a Bundesliga side against a league leader this millennium, equalling the score achieved by Bayern against Borussia Dortmund in April 2019.

Crucially for the host, it also brought an end to a six-match winless streak across all competitions and marked only the third league win for Leverkusen this season, leading Alonso to praise his side's performance.

"The first half was very close, neither side had many chances and it wasn't easy to attack. Union are standing so high in the table for a reason," he said. "In the second half we wanted to stay focused. We had more spaces, our fast players could run deep and were very good at finishing. We are very happy today.

"Today was important. We want to continue in the same way. We always need that mentality to attack and score goals, to stay hungry.

"The 2-0 made the big difference. We defended going forward, the pressure from Jeremie Frimpong was good. That's the mentality we want to have."

Alonso's side has two Bundesliga matches against Koln and Stuttgart before the World Cup in Qatar, with the league campaign set to resume on 22 January.