In 2011, Novak Djokovic played one of the greatest seasons in tennis history -- one that began with a 41-match winning streak and ended with three Grand Slam titles. And yet that's nothing compared to what he has accomplished in 2015.
At a time prime for letdown, Djokovic has lost just one match since the US Open, his 10th major title. He has won titles in Beijing, Shanghai and Paris with the possibility of another to come at the ATP World Tour Finals.
Four years ago, after his triumph in New York, Djokovic didn't win another tournament in 2011 and failed to make it out of the round-robin stage in the Tour Finals. A victory in London would break new ground -- he would become the first man in history to win four successive year-end titles, and he'd lock up his 11th championship of the season, one more than in 2011.
In many ways, Djokovic's 2011 season was a breakthrough year for him, even though he had already won a major at the 2008 Australian Open. It was about Djokovic showing he was no longer playing third string in an era dominated by Roger Federer and Rafael Nadal.
This season, though, has been about Djokovic reasserting and strengthening his dominance over the rest of the men's game. It's not just that the Serb has been winning tournament after tournament, but the manner in which he has been sailing to victory.
Only once during Djokovic's run to the Beijing title did he drop as many as three games in a set, and only once on the way to winning Shanghai was he extended to a tiebreaker. Heading into this week's Tour Finals, Djokovic has conceded only one set since the US Open, which was against Stan Wawrinka in the semifinals of last week's Paris Masters.
"Somehow I feel that this [season] is better," Djokovic said to the press at the China Open. "I just feel like I'm a more complete person and player. I'm a father and husband, just different circumstances that happen in my life in the last couple of years that have helped me to enjoy all the success that I have had on the tennis court even more.
"So 2011 was absolutely an incredible year, but I feel like this year I'm enjoying it even more because I'm more, I would say, fulfilled, more complete, more mature as a person and a player."
If there were subtle weaknesses in Djokovic's tennis in 2011, there are none now.
"All of the pieces of the puzzle are coming together at the right time," Tim Henman, a former six-time Grand Slam semifinalist and current BBC analyst, told ESPN.com earlier this month. "Since Djokovic started working with [Boris] Becker, he has definitely looked more comfortable at net. The biggest element that has changed in Djokovic's game is his serve. ... His motion now is so much better. His serving is so consistent and that allows him to dominate from the back of the court.
"Djokovic is the best mover that I've ever seen, and mentally he's so much more resilient than he was before," Henman added. "A few years ago, there was a bit of a question mark over him as he had pulled out of a lot of matches and you felt that that there were some physical issues that plagued him mentally. But no longer."
Even in the unlikely possibility that Djokovic were to fail to qualify for the semifinals in London, he will still close out the year as the world's No. 1 player, holding a winning percentage north of 90 percent, right in the ballpark of the sport's best annual runs (John McEnroe in 1984, Jimmy Connors in 1974 and Federer's ridiculous seasons in 2005 and 2006).
Despite everything he has accomplished, Djokovic's desire to improve keeps him motivated.
"Ten years ago, when I started playing professional tennis, the game was completely different," Djokovic recently told ESPN.com. "Now, Nadal, Federer and [Andy] Murray and some others, including myself, have taken this sport to a higher level and raised the standard. And that's exciting seeing how far we can take the level of this game, seeing how far we can go."