LONDON: Roger Federer and Rafael Nadal have utterly dominated during 2017 but as the season ends there is a palpable sense that the top of the men's game is in a state of rare flux.
Federer and world number one Nadal rolled back the years to reclaim the top two ranking spots and hogged the four Grand Slam titles as Novak Djokovic and Andy Murray slipped off the radar.
But the season ended with a new name on the ATP Finals trophy after Grigor Dimitrov triumphed in London.
So with a clutch of new faces in the top 10, are we witnessing a changing of the guard or just a blip?
Roger and Rafa show
Can ageing Federer and Nadal still keep up the pace? Federer, 36, shocked observers by returning from a six-month injury layoff to win the Australian Open, beating Nadal in a thrilling five-setter.
That set the stage for a year of Grand Slam domination, with Nadal picking up his 10th French Open and winning the US Open for the third time while Federer won a record eighth Wimbledon Crown.
Nadal, 31, pulled out during the Paris Masters with a right knee injury and was forced to call it quits after the first round in London but sounded upbeat about his chances of being ready to compete at the start of next season.
Federer looked short of his best during his semi-final defeat in London at the hands of David Goffin but stayed positive, saying he had had an "amazing year" as he plotted for next season.
Neither sound like they have plans to go anywhere yet.
Can Djokovic, Murray recover magic?
Both Djokovic and Murray were forced to wait their turn to taste success but they eventually made up for lost time.
Twelve-time Grand Slam winner Djokovic, currently ranked 12th, is set to make his return in Abu Dhabi late next month after six months out following an elbow injury, with questions over whether he has the motivation to dominate tennis again.
Murray has dropped to 16th in the rankings after months out with a hip injury but he remains focused on returning to action in Australia after Christmas.
Federer expects Djokovic, Murray and Swiss compatriot Stan Wawrinka to return and provide a sizzling start to 2018.
"I expect greatness from them," said Federer. "I wouldn't be surprised if it worked out for them like it worked out for me and Rafa.
"I also hope Kei (Nishikori), Tomas (Berdych) and Milos (Raonic) all find their way back on tour ... in Australia because that would make it a quite epic comeback.
"Then you mix them together with the new guys ... It could be a very cool start to the year."
Can Dimitrov build on ATP Finals win?
Winning the biggest title of his career has propelled Bulgaria's charismatic Dimitrov to the dizzy heights of third in the world and given him high hopes for 2018 after his best ever season.
The 26-year-old has never gone further than the semi-finals at a Grand Slam, and there is a danger he could be part of a generation squeezed between the "Big Four" and an up-and-coming younger group.
But he is eyeing even bigger prizes next season after his breakthrough win in London.
"One of my main goals is to win ... a Grand Slam tournament. This has always been a dream of mine," said the Bulgarian.
Alexander Zverev, 20, is a man in a hurry. The German had a season to remember with five trophies, including two Masters titles.
The leader of a pack of richly talented youngsters that includes Australia's Nick Kyrgios and Canadian teenager Denis Shapovalov has shot to world number four even though he disappointed in London.
"It's been an awesome year," Zverev said. "Still, the end of the year was absolute crap for me. If I played the whole year like I did (at the end) ... I don't think I would have finished top 50.
"But that's OK. I'm going to go on holidays now. Then I'm going to work hard in the off-season."