Rafael Nadal ensured he will end the year as world number one with victory over Stanislas Wawrinka at the ATP World Tour Finals in London.
The Spaniard, 27, won 7-6 (7-5) 7-6 (8-6) to secure a semi-final place and guarantee he cannot be overtaken by world number two Novak Djokovic.
It will be the third time Nadal - who called it "the perfect scenario" - has finished a year top of the rankings.
Tomas Berdych beat David Ferrer 6-4 6-4 in the day's other Group A match.
Nadal has admitted he doubted whether he would ever recapture his best form when he was forced out of the sport for over seven months with a serious knee injury.
Since returning in February, his results have been spectacular.
He arrived in London with a career-best record of 72 wins and six losses for the year, 10 tournament victories and Grand Slam titles at the French and US Opens that took his tally of majors to 13 - four behind all-time leader Roger Federer.
"Today it was important to secure the number one at the end of the season," said Nadal.
"During all the season I said it's not my goal, but the real thing is after all the success I had, after all that happened last year, I felt I deserved to be there at the end of the season.
"It's one of the best things that I did in my career - to come back to number one after three seasons. That's very difficult in our sport, and after a very important injury.
"That's an emotional thing for me, for sure."
Two straight-sets wins within 24 hours have seen Nadal qualify from the group as efficiently as possible, and he remains on track to land the most significant title still absent from his CV.
Wawrinka, 28, might be enjoying the best year of his career but Nadal remains too great a challenge for the Swiss, who has now lost all of their 12 matches without winning a single set.
He got close to breaking that duck at the 25th and 26th attempts, but Nadal once again proved himself to be a supreme competitor.
Under pressure at 5-5 in the first tie-break, the Spaniard played a brilliant defensive backhand that dropped on the baseline and Wawrinka pulled a backhand wide to offer up set point.
When Wawrinka stumbled at the net, the world number one swooped in to fire a forehand past him and take a grip on the match.
The frustration of yet again failing to win a set after 61 minutes of hard work boiled over and Wawrinka was warned for racquet abuse as he headed to his chair.
In a tense start to the second set he complained to umpire Carlos Bernardes about the noise coming from the watching Nadal camp. However, the official took no action despite admitting it was "not normal".
Nadal needed no help from his camp or his large following among the 17,000 spectators to get the key break in game four as Wawrinka dumped a volley in the net and made two forehand errors.
The Swiss challenge looked all but over at 4-1 down, but Wawrinka was still creating opportunities and, after a couple of suspect volleys earlier in the match, a kind net cord helped him get back on level terms.
Wawrinka was playing the more spectacular winners but, just as in the first set, it was Nadal who prevailed when the pressure was really on in the tie-break.
A smash saved set point and the Spaniard clinched victory after two hours and 12 minutes when Wawrinka made his 23rd forehand error of the match.
Asked about his complaints to the umpire in the second set, Wawrinka said: "It's nothing personal against Rafa or against Toni [Nadal, coach and uncle].
"We all know, players and umpire, that Toni is always trying to help Rafa. That's normal. That's part of the game. But when it's too much, it's too much.
"Today I didn't agree with the umpire that he didn't tell him something or he didn't give him second warning just because it was Rafa."