(CNN) - The ATP Finals is heading into the homestretch in London. Hosting the year-end men's extravaganza since 2009, the tournament moves to Turin, Italy from 2021.
It's anyone's guess as to how many of the Big Four will still be playing then.
All four are very much around in 2019 -- after Andy Murray thought his career might be over.
While the Scot won't be competing at the O2 Arena, Rafael Nadal, Novak Djokovic and Roger Federer will be, joined by ample youth.
The ATP Finals is set to determine who'll end the year at No. 1, either the ailing Nadal or Djokovic.
Nadal landed in Group Andre Agassi, alongside Stefanos Tsitsipas, Daniil Medvedev and Alexander Zverev, while Djokovic, Federer, Dominic Thiem and Matteo Berrettini feature in Group Bjorn Borg.
Here are the four biggest storylines of the event, which starts Sunday.
The most relevant question relating to Nadal in recent ATP Finals has not been, 'Will he win it for the first time?' but rather 'Will he play?'
The array of injuries that have affected the 33-year-old Spaniard have surfaced throughout seasons but seem to especially derail him near the end, by which time he's piled up matches on his beloved clay.
Nadal missed the ATP Finals in 2012, 2014, 2016 and 2018 -- and played only one match in 2017 prior to pulling out courtesy of a right knee complaint.
After getting married to longtime girlfriend Xisca Perello last month -- he was the last member of the Big Four to tie the knot -- everything seemed fine on the health front.
But Nadal sustained an abdominal injury, which affects his serve, warming up ahead of his semifinal at the Paris Masters last week and had to withdraw.
He hopes to play but it remains to be seen whether he'll be fit enough to take to the court against Zverev on Monday.
"I need to see how things evolve every single day," Nadal said in Friday's pre-tournament press conference. "I have good hopes to be 100% ready for Monday.
"For me the main goal is to be healthy. I really hope I will be able to serve and every single day will feel better."
The chase for No. 1
Why would Nadal play the ATP Finals and risk making his injury worse, which could have a knock-on effect and spill over into his off-season, then January's Australian Open? In Melbourne, Nadal could match Federer's all-time men's record of 20 majors.
Perhaps because Nadal and Djokovic are locked in a battle for the year-end No. 1.
Nadal reclaimed No. 1 this week after an absence of 12 months, replacing Djokovic.
Nadal is bidding to become the year-end No. 1 for a fifth time, which would match Djokovic and Federer. Djokovic would tie Pete Sampras for the all-time ATP lead if he gets to six.
But as much as that would provide motivation for Nadal, 10 years ago it might have been more of a goal. Now, winning the ATP Finals for the first time could be the even bigger target.
"For me personally it wasn't one of my goals," Nadal said of the year-end No. 1 ranking. "I'm super happy to be where I am today but for me the goal is to play the full tournament with full health."
When one ponders Nadal's achievements in the game -- even if also considering his struggles playing indoors -- it's remarkable that his record at the ATP Finals is a mediocre 16-13.
The Next Gen is here
Thinking ahead to when the Big Four are no longer around, the ATP launched its Next Gen marketing campaign three years ago. It also featured the creation of a new tournament for players under 22, the Next Gen ATP Finals in Milan.
Well, the Next Genners truly arrived in 2019.
Four players 23 or under qualified for London in debutants Medvedev, Tsitsipas and Berrettini, while Zverev returns to defend his title.
The last time that happened? In 2009 when it was the standout quartet of Nadal, Djokovic, Murray and Juan Martin del Potro.
Tsitsipas started the year the strongest, making the Australian Open semifinals as part of a season in which the flashy all-rounder beat Federer when the Swiss was the two-time defending champion in Melbourne; downed Nadal on a clay court; and rallied from a set down against Djokovic at the Shanghai Masters.
Djokovic had been 60-1 in China -- where he enjoys massive crowd support -- when claiming the first set.
Medvedev -- the counterpuncher with a huge serve -- ended the strongest. The 6-foot-6-inch Russian transformed from villain to crowd favorite at the US Open -- part of a stretch where he made six consecutive finals -- and almost pulled off a monumental comeback against Nadal in one of the most memorable grand slam finals in recent history.
The calm, big-serving Berrettini's semifinal showing in New York helped him become the first Italian man to reach the year-end championships since Corrado Barazzutti in 1978.
Zverev's appearance in the final in Shanghai contributed his inclusion, although the German was the lone member of young quartet not to make a grand slam semifinal.
Federer bids for lucky No. 7
Given his record eight titles at Wimbledon, Federer's memories of London are, surely, mostly uplifting. Yet his two most painful defeats in the final of grand slams have come at Wimbledon, when he was dethroned by Nadal in five sets in 2008 in what many consider to be the greatest match of all time and only five months ago when Djokovic upended the now 38-year-old.
While not matching the 2008 epic for quality, Djokovic saving a pair of match points and prevailing in the first fifth-set tiebreak in Wimbledon history made it one of the most dramatic contests ever.
A record-extending seventh title at the ATP Finals wouldn't banish memories of the Djokovic reverse but it would be the best possible way for Federer to end his season, prior to embarking on a handful of exhibitions in Chile, Argentina, Colombia, Mexico City and China to end 2019.
With Djokovic and Federer in the same group, there's a high likelihood of the pair meeting twice -- in the round robin, then in the final -- as was the case in 2015.
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