Bayern Munich striker Robert Lewandowski admits he found life tough under Jurgen Klopp at Borussia Dortmund.

The Pole will take on Klopp's Liverpool side next week in the Champions League last-16 tie, with the scores at 0-0 after the first leg at Anfield.

"The first two years at Borussia Dortmund, my finishing was not like it is now," said Lewandowski.

"After every single training session I stayed on the pitch and practised with my left foot, my right foot and being systematic and repeating the movements was very important.

"It did not matter what I did with the team, I knew I had to stay after training to do things for myself.

"Sometimes I did it without a goalkeeper, who might have been tired or not have time for me, because I would stay on for two extra hours.

"I remember the first six months at Dortmund, the training was very hard under Jurgen Klopp. I was very young and wanted to show everyone I could give 110% in every training session.

"After two or three months, I was very tired and could not show all my skills. It was too much for me and I needed three months lighter training to get back to form.

"When I think about it now, I learned a lot.

"After the first few months my body was ready to work harder. Around November I scored a lot of goals and my team-mates said 'Robert is back', and I knew then I could show all my skills."