Borussia Dortmund Jurgen Klopp hailed a \"perfect\" day as his side opened up a seven-point lead at the top of the Bundesliga. The Westphalians beat Mainz 2-1, while title rivals Bayern Munich went down 2-0 at Bayer Leverkusen. Forward Mohamed Zidan, who left Dortmund in January, scored the Mainz goal and Klopp could even afford to be pleased for his former player. \"We have won and he has scored - that\'s perfect,\" Klopp told Sky. \"In truth, I hadn\'t realised he had scored, otherwise I would have said something to him. I think we have all won something today.\" Klopp joked before the game he and his team had been drinking champagne on the way to the stadium after hearing of Bayern\'s defeat. And his team certainly believe they can go on to retain their title. \"Last year, we had a double-figure advantage at times and everybody said we would fall apart at some stage, but we didn\'t,\" said defender Mats Hummels. \"Therefore, I don\'t know why this year should be any different.\" Goalkeeper Roman Weidenfeller added: \"Everything is realistic, but we just want to simply carry on playing our simple football.\" Mainz forward Zidan will also celebrate if Dortmund win the league this season since he too is in line for a winner\'s medal and bonus, having spent the first six months of the season with them. However, he found little to celebrate from his fifth goal in as many games since joining Mainz. \"I respect Dortmund a lot,\" he said. \"I played three-and-a-half years there and it was a wonderful time for me. Dortmund will always have a place in my heart and I promised that I would be respectful to the club and the fans if I scored.\"