Juan Martin del Potro believes that he needs to play better than he did against Roger Federer to progress to the final of the ATP World Tour Finals in London.
The 24-year-old defeated the Swiss on Saturday to set up a clash with Novak Djokovic in the semi-finals of the tournament.
"We (he and Djokovic) played each other many times," BBC Sport quotes him as saying.
"I beat him in the Olympics, he beat me in the US Open quarters - but I played a good match there. [Sunday] will be a big challenge against him.
"I need to play better [than against Federer] if I want to beat him, but it's not going to be easy."
In the other semi-final on Sunday, Andy Murray will face Federer.
