When Roger Federer trudged off the court at O2 Arena late Saturday night, a dramatic semifinals winner over Stan Wawrinka, he looked weary and grim. The 33-year-old probably already knew that he wasn't going to play the Sunday final against Novak Djokovic.
About 30 minutes before the anticipated match was scheduled to begin, Federer stunned the tennis world by announcing on his Facebook page that he was withdrawing.
"I am sorry to announce that I cannot play the finals tonight vs. Novak. I hurt my back late in the match yesterday against Stan," the post said. "I am very disappointed and I hope to be feeling better soon.
"It's been an extremely difficult decision because I love playing in London and the ATP World Tour Finals have been an absolute highlight of my career. Unfortunately, my back problem does not allow me to play right now. I hope all tennis fans and those involved in the event will understand."
It might have been the most dramatic walkover in the sport's history.
The pressing question: What's his status for the Davis Cup final, which begins Friday and features the Swiss team of Federer and Wawrinka against the French trio of Jo-Wilfried Tsonga, Richard Gasquet and Gael Monfils?
Did Federer withdraw because he wasn't capable of playing against Djokovic -- or to give himself a chance to be ready for Friday's first day of singles play in Lille, France, on red clay? Do not forget that this is the one meaningful tennis trophy he's never won.
Federer's aching back, a problem in recent years, has been blamed for his uncharacteristically poor performance in 2013.
It was only the third time Federer has granted a walkover to an opponent. Both times prior it was due to his back, in 2012 against Tsonga in Doha and to James Blake at the 2008 Paris Indoor event.
This was the first finals walkover in the 45-year-history of the ATP's year-end event.
In 1,221 career matches, Federer has famously never retired from a match once it started. With that kind of record, it's easy to take his recent performance for granted. Since turning 33, he has won 32 of 35 matches and was looking for his record seventh year-end title. It was 11 years ago to the day that Federer beat Andre Agassi for his first year-end championship at the Westside Tennis Club in Houston.
In the larger context, it was fitting that these two reached the final match of the ATP season, for Djokovic and Federer (despite failing to win a major) played the best sustained tennis. They were first and second in the rankings, of course, but also in the critical categories of titles, match wins, top-10 wins and match-win percentage.
It had to be a deeply disappointing turn of events for Djokovic, who had won 31 consecutive indoor matches. This is his third consecutive year-end title, something that hasn't happened since 1985 to 1987, when Ivan Lendl scored the three-peat.
And so this strangely disjointed ATP season abruptly ends. There were two first-time Grand Slam singles winners this year, Wawrinka and Marin Cilic, and 2015 could offer more gate-crashers. Milos Raonic, Grigor Dimitrov and Kei Nishikori will make their bids for a major title.
That said, Djokovic is already the considerable favorite to win the title in Melbourne when the Australian Open begins two months from Wednesday.
It's going to be a big year for Djokovic. He has been the year-end No. 1 three times in four years, but it's one thing to win titles indoors in the fall and quite another to hoist major trophies. Djokovic has come out of the box quickly in the past, winning the Australian Open four of the past seven years. But outside of his magnificent run in 2011 -- when he won three of four Grand Slam singles titles -- Djokovic hasn't dominated in quite the fashion his skill set would suggest.
With Federer still aging gracefully, Rafael Nadal struggling with a series of troubling injuries and Andy Murray wrestling with post-Lendl confidence issues, there is a vacuum at the top of men's tennis.
Djokovic seems best equipped to fill it.
Bros go out with a bang
They turned 36 this season, but Bob and Mike Bryan continue to dominate men's doubles.
On the day that 33-year-old Federer withdrew from the Barclays ATP World Tour Finals singles finale, the Bryans won their fourth year-end title.
The California twins came from behind to defeat the No. 7-seeded team of Ivan Dodig and Marcelo Melo 6-7 (5), 6-2, 10-7.
It is their 103rd team title, and this is the fourth year they have collected double-digit titles. They finished the season strong, reaching the final at Wimbledon, winning the US Open and London titles -- and producing a record of 64-11.