Juan Martin Del Potro admits he faces a tough task if he is to qualify for the semi-finals of the ATP World Tour Finals.
The sixth-seeded Argentine claimed his first win of the competition on Thursday as he easily saw off Serbia's Janko Tipsarevic 6-0 6-4 at the O2 Arena.
However, he has to face six-time winner Roger Federer in his final Group B match, with the Swiss player looking in imperious form in his two contests so far.
Del Potro also has the disadvantage of a head-to-head loss against David Ferrer, who just needs to beat the ailing Tipsarevic in order to progress.
"Janko was fighting until the last game, he is a very good player. Now it is a very high step for me on Saturday against Federer but I will try to win," Del Potro told Sky Sports.
Del Potro is returning to the season-ending competition for the first time since 2009 as he has finally re-joined the tennis elite following a lengthy lay-off with a wrist injury.
"I worked a lot every day to be here. I had a really good comeback last year and now I'm in the top 10. I'm so happy to play tennis again and I really want to enjoy this week," he said.
He added: "I'm so glad to be one of these eight players, it's an amazing tournament and I have good memories from 2009 when I made the final. Now I'm trying to play my best tennis."