ATP World Tour Finals: Magic No. 7 for Roger Federer or Rafa Nadal’s first?

After one of the most surprising tennis seasons of all time, who will take home the season-ending ATP World Tour Finals title?

The tournament for the top eight singles and doubles players on the men’s tour starts Nov. 12 at London’s O2 arena.

World No. 1 Rafael Nadal leads an eight-man field that also includes six-time winner Roger Federer, debutants Alexander Zverev, Grigor Dimitrov, David Goffin and Jack Sock and former qualifiers Dominic Thiem and Marin Cilic.

There is $8 million up for grabs in prize money, including a cool $2.6 million for an undefeated champion.

Nadal’s group contains Belgium’s Goffin, Thiem of Austria and Bulgarian Dimitrov, while Federer’s section includes American Sock, Germany’s Zverev and Croatian Cilic. Two players from each group qualify for the semis with the winners meeting in the final on Nov. 19.

Here are the main storylines to follow:

Lucky No. 7 for Federer?

Down and out a year ago with a knee injury many feared could signal the end of his record-breaking career, Federer staged one of the most surprising and remarkable sporting comebacks in January by winning the Australian Open title in Melbourne.

With Novak Djokovic and Andy Murray struggling with injuries throughout the season, the Swiss added a seventh Wimbledon title to take his total tally in the majors to 19.

Having come within a whisker of clinching the No. 1 ranking at the grand old age of 36 this fall, Federer is the favorite to win a record-extending seventh title at the ATP Finals.

With four-time winner Djokovic and defending champion Murray both out for the season with injuries, few who will be able stop the Swiss maestro if he is fully healthy on one of his favored surfaces.

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Nadal aims for first title

After losing the Australian Open final to Federer, Nadal’s season really clicked into gear on the clay, where he become the first player to win the same major 10 times at Roland Garros in Paris in June.

The 31-year-old Spaniard then knocked an injured Murray off the No. 1 spot in August before adding a third US Open title in September.

Nadal may have won 16 grand slam titles, two Olympic gold medals and four Davis Cups, but he has yet to win the ATP Finals.

This season, 2010 finalist Nadal has said winning the ATP title in London is one of his main goals. With his confidence sky high and Murray and Djokovic out, he will surely fancy his chances this time around.

Can the young guns spoil the party?

With all eyes on Federer and Nadal, could Zverev or Thiem upstage the two superstars?

Nicknamed “The Dominator” for his powerful single-handed backhand, 24-year-old Thiem is playing in the ATP Finals for the second year in a row.

Last year, he left London after the group stages with one win and two defeats, but not before meeting with Jose Mourinho, who used to coach his favorite Chelsea team, who last season won the English Premier League.

Could Thiem emulate Chelsea and clinch the ATP Finals title? It remains to be seen because this two-time French Open semi-finalist excels on clay.

Playing in his first ATP Finals, 20-year-old Zverev is the youngest qualifier for the event since former US Open winner Juan Martin Del Potro in 2008.

Six-foot-six (1.98m) tall and armed with a blistering serve and lethal double-handed backhand, Zverev won five titles this season. These included Rome, where he beat none other than Djokovic on clay, and Montreal, where he claimed the scalp of Federer on hard court.

Having moved from No. 24 in the rankings at the start of the season to No. 3 behind Nadal and Federer, Zverev has made huge strides this year and is more than capable of springing a huge surprise in London.

Could we see an outside winner?

With Murray, Djokovic and three-time grand slam winner Stan Wawrinka all absent and four men playing the finals for the first time, could we see an upset this year in London?

It remains to be seen how far Belgium’s Goffin and Sock, the first American in the field since Mardy Fish in 2011, will go after qualifying at the last minute.

But has Dimitrov’s time finally come?

At the start of his career, the popular Bulgarian was likened to Federer for his elegant single-handed backhand. Then he became best-known for being the boyfriend of five-time major winner Maria Sharapova.

Although he reached the Wimbledon semi-finals in 2014, his form soon fizzled out.

But this season, the 26-year-old is finally making a name for himself with consistent results that have earned him his first appearance at the ATP Finals.

Having hired Dani Vallverdu, one of the Murray’s former coaches, in July last year, Dimitrov worked hard on improving his fitness during the off-season.

The results have paid off as he went 16-1 at the start of the season in Australia before being stopped by none other than Nadal in the Australian Open semifinals.

With three titles under his belt this year, he heads to London at a career-high ranking of No. 6. Don’t be surprised to see him go far.