"It wasn't a bad season in Germany but now I am coming back to conquer England," he told a newspaper in Kiev.
"I'm a person who likes to face new challenges. It's time to prove to myself and to those people who think I can score a lot of goals only in Bundesliga, that it's not so.
"Liverpool is one of the world's strongest clubs. To play for it is a great honour for any footballer.
"I know it's not going to be easy but I feel ready for a real fight to get into the first choice team."