World number one Rafael Nadal remains on track for a first ATP World Tour Finals title after beating Roger Federer to reach the final.
The Spaniard, 27, won 7-5 6-3 in London to claim his 22nd victory in 32 meetings between the pair.
Nadal has reached the final at the season-ending tournament just once before, losing to Federer in 2010.
He will play Novak Djokovic in Monday's final after the world number two beat Stanislas Wawrinka 6-3 6-3.
This clash between Nadal and Federer attracted another sell-out crowd of more than 17,000 spectators to the O2 Arena.
Federer might have slipped to seventh in the rankings this year, but a 4-0 record against Nadal indoors gave the Swiss star's followers cause to hope.
The 32-year-old made a better start than he had against Juan Martin Del Potro on Saturday but could not convert any of three break points in game six, and the inconsistency that has plagued him in 2013 returned.
A poor sliced approach was swatted away by Nadal's fearsome forehand as the Spaniard broke for 5-4, only for Federer to hit back immediately with a brilliant forehand of his own in a stunning rally as he made it 5-5.
The crowd were thrilled by the unexpected comeback but Federer could not sustain it, handing over another break with a forehand in the net, and Nadal ruthlessly converted his second chance to close out the set.
Federer had admitted on Saturday that his once unrivalled forehand had "gone a bit up and down" recently, and he blasted a couple of tempters way over the baseline in trying to pressure Nadal.
Instead, it was the Spaniard who turned the screw midway through the second set with a backhand winner down the line, and Federer buried a forehand in the net on break point.
The Swiss shook his head at another mistake that took his error count to double that of Nadal's, and a wayward volley on match point handed the top seed an astonishing 75th win and 14th final appearance of the year.
"The most satisfying thing always is the tournament more than the personal victories," said Nadal.
"So for me it's more important to be in the final in the last tournament of the year than having the chance to win against Roger.
"To be able to win the final is much more important than any victory against any opponent."
Federer made 32 errors to 14 from Nadal, and the Swiss admitted: "I just struggled to stay consistent enough throughout the match, and that's why he deserved to win.
"I thought the margins were more on his side. He was playing more consistent. He was playing more solid. He plays the way he always plays.
"I just couldn't come up with the shots when I needed them, forehand or serve, moving forward."