The PM revived a line used during her Tory leadership campaign to respond to claims the two clashed over dinner.
She also declined to commit to settling the issue of expats' rights by June.
EU sources claim UK misunderstanding of the talks process, and ignorance about how Brussels works, could lead to no deal being agreed on the UK's exit.
According to German newspaper Frankfurter Allgemeine, the prime minister and Mr Juncker clashed last Wednesday at Downing Street over Mrs May's desire to make Brexit "a success" and whether the issue of protecting the rights of expat UK and EU nationals could be agreed as early as June.
Speaking to BBC political editor Laura Kuenssberg, Mrs May said there were a lot of similarities and common ground in the two sides' positions.
She added: "But look, I think what we've seen recently is that at times these negotiations are going to be tough.
"During the Conservative Party leadership campaign I was described by one of my colleagues as a bloody difficult woman. And I said at the time the next person to find that out will be Jean-Claude Juncker."
Read More (...)