Novak Djokovic will face a rejuvenated Rafael Nadal in the ATP World Tour Finals last four in London on Saturday.
The pair will meet for the 46th time, with Nadal leading 23-22, in the first semi-final at 14:00 GMT.
"Against Novak, I know what I have to do," said fifth seed Nadal. "But it's going to be very difficult to make it happen. It's a big challenge."
Roger Federer plays fellow Swiss Stan Wawrinka in the second semi-final at 20:00 GMT at the O2 Arena.
Wawrinka knocked out Britain's Andy Murray by winning the final group match on Friday.