LONDON -- They're still in their 20s, yet their rivalry is already one of the most heralded in men's tennis history.
Rafael Nadal vs. Novak Djokovic has become a classic.
The pair met for the first time at tour level seven years ago and, when they go head-to-head in the final of the season-ending ATP World Tour Finals on Monday, it will be for the 39th time.
The Spaniard and the Serb have been dominant this season, with Nadal replacing Djokovic for the No. 1 ranking last month on the back of a stunning comeback from a career-threatening knee injury.
"Whenever we play each other, it's a huge challenge," the 26-year-old Djokovic said. "This is the probably the most competitive tournament that we have after Grand Slams in our sport, and we both want to crown this season in the best possible way and end it with a title."
Djokovic extended his winning streak to 21 matches since losing in the U.S. Open final to Nadal when he beat Stanislas Wawrinka 6-3, 6-3 in the semifinals. Nadal had earlier dispatched his old rival Roger Federer in straight sets.
The world's two highest-ranked players are unbeaten this week and have faced each other five times this season, with Nadal leading the 2013 meetings 3-2.
After losing a set in each of his three previous matches, Djokovic raised his game on Sunday, making only 14 unforced errors as he broke Wawrinka four times.
"The performance was definitely by far the best I've had in the tournament," said Djokovic, who is bidding for a third title at the elite, season-ending event and has been the best player of the indoor season.
Nadal ended Federer's hopes of finishing a disappointing season on a high note, defeating the six-time champion 7-5, 6-3. He leads Djokovic 22-16 in their career head-to-head record, although he lost their previous encounter last month in Beijing soon after regaining the top ranking.
"After the U.S. Open final, I haven't lost a match," Djokovic said. "I'm trying to take a lot of positive energy out of this confidence and out of this run. I won against him in the finals of Beijing in straight sets and I played a great match. I liked the way I improved in our matches. I like that I changed a few things that allowed me to win against him. That's something that I will definitely try to use tomorrow. "
Nadal, who claimed his first win over Federer on an indoor hard court, extended his winning career record over the 17-time Grand Slam winner to 22-10 and has now beaten the Swiss player on every surface. Federer had won their four previous matches at the ATP Finals, dropping only one set.
"The most important thing for me is (that indoor) is the toughest surface for me to play," Nadal said. "The most difficult for me was to be able to win four matches against top eight players. It's a very good way to finish the year."
Chasing his first title at the Finals, Nadal played down the importance of the victory over his greatest rival.
"It is probably one of my best seasons," said Nadal, who returned to the tour in February after a seven-month injury layoff. "The most satisfying thing is always (to win) the tournament more than the personal victories. So for me it is more important to be in the final in the last tournament of the year than to have the chance to win against Roger."
Since returning from injury, Nadal has won 75 matches --losing only 6 --to win 10 titles including the French Open, the U.S. Open and five Masters 1000 events.
Federer's season was very different. Hampered by a back injury, he won only one title in 2013 and was hoping to secure a major trophy before the year-end break.
He played aggressively early on but faded after losing the first set, making too many mistakes to threaten the Spaniard.
"In the beginning, his serve worked very well," the 27-year-old Nadal said. "My feeling is that in the first set, he played very well. He was closer than me to have the break. So in my opinion until 4-all, he was playing better than me."
Nadal and Federer treated the O2 Arena fans to some superb exchanges early on, reminiscent of the classic matches that have punctuated their nine-year rivalry. But Nadal was much sharper on the important points, converting all four break points he earned. Federer had 32 unforced errors and made some uncharacteristic poor volleys.
Federer, who beat Juan Martin del Potro on Saturday to qualify for the semis while Nadal had a rest day, also appeared to fatigue near the end.
Federer failed to convert three break points in the sixth game before Nadal took advantage of his first opportunity to break for 5-4 with a superb forehand winner.
Federer broke back immediately, but failed to build on the momentum. He dropped his serve in the very next game before conceding the set. He then struggled with his rhythm and was broken twice in the second set.
"I struggled to stay consistent enough throughout the match, and that's why he deserved to win" Federer said. "This game is a game of making or missing your opportunities."