Novak Djokovic battled through "an incredible fight" with Taylor Fritz on Saturday, shrugging off physical discomfort to win 6-4, 7-6 (8/6) and set up a blockbuster Shanghai Masters final with world number one Jannik Sinner.
Earlier the Italian beat Tomas Machac 6-4, 7-5, ensuring in the process that he will finish the year at the top of the rankings.
"I won't be a favorite on the court, but hopefully I'll be able to come out physically fresh enough to challenge him for what may be a long match," said Djokovic of Sunday's final.
"I have to expect probably the toughest encounter of the tournament, and I have to be ready for it."
Sinner said playing Djokovic would make the final "even more special".
"It's going to be a very tough one, it's one of the biggest challenges we have in our sport," he said.
Fighting spirit
Djokovic has now beaten the United States Fritz at each of their 10 encounters.
The Serb broke the 26-year-old US Open finalist in the fifth game to win the first set in 38 minutes.
But the second set lasted twice as long, and was a much more tense affair, as the players sent each other scrambling across the court in a series of thrilling rallies.
The 24-time Grand Slam champion called it an "incredible fight".
"He could have easily won the second set... In the tiebreak it was really anyone's game, (but) I feel like I was never giving up," Djokovic said.
In the second set, after an almost 13-minute fifth game eventually held by Fritz, Djokovic bent over in exhaustion, breathing heavily.
The world number seven missed a good chance in the eighth game when he failed to convert a break point, with Djokovic recovering to hold.
"I just never really came up with what I needed to come up with in a lot of big moments and the big points," said the American.
Djokovic came from 3-5 down in the tiebreak to level and then win after three unforced errors from Fritz.
The 37-year-old, who has been wearing a brace on his right knee and has been having trouble with his left one too, had to take a medical time-out for his left leg toward the end of the second set.
However, he said after the match none of the issues were serious.
After physio, "I could move freely and run and sprint, which is good news", he said.
"I guess at this age you kind of have to expect that each match something different happens physically."
Sinner on top
The 23-year-old Sinner has been top of the world rankings since June and after his ruthless semi-final victory will stay there until at least the end of the season, the first Italian to do so.
"It was only a dream just to become number one... Now finishing (the year there) is also a different feeling," the Italian said.
"It's a good feeling. I'm very happy to achieve this. It was an amazing season for me and it's not finished yet," he said, adding that, for now, he was "focused on tomorrow".
Sinner's meeting with Machac got off to an electric start when the Czech broke in the first game of their semi-final.
The 33rd-ranked Machac was looking to repeat his giant-slaying of Thursday when he knocked out world number two Carlos Alcaraz in straight sets.
But two-time Grand Slam champion Sinner kept his cool to break back in the fourth, then claimed the tenth game to clinch the set.
In the second set neither was able to break serve until the 12th game, when the Italian hit a winner to book his place in Sunday's final.
"It was very tough... he is playing with so much confidence and he can hit a ball really hard, and is serving very well," said Sinner.
"I tried to stay there mentally... I'm very happy about how I'm fighting at the moment."