Novak Djokovic saw off Japan's Kei Nishikori in three sets to reach the final of the ATP World Tour Finals.
The world number one, chasing a third straight season-ending title, came through 6-1 3-6 6-0 at the O2 Arena.
Djokovic recovered his form superbly after reacting badly when the crowd at the O2 Arena in London applauded a double fault in the second set.
The Serb will face Roger Federer or Stan Wawrinka in Sunday's final, with the Swiss pair meeting at 20:00 GMT.
"I found it a little bit difficult mentally to stay concentrated throughout the whole match," said Djokovic.
"After the emotional three matches I had, especially on Friday when I achieved the goal to finish as number one of the world. Knowing that, I felt a little bit flat emotionally.
"I needed a little bit more time to give myself a boost."
Djokovic, 27, picked apart the Nishikori game for the opening 25 minutes as he relentlessly scraped the lines with his returns.
Under constant pressure, the Japanese player saved one break point in game four but planted a makeable volley wide on the second.