Here's what to expect from Nadal, Federer and the elite eight at the World Tour Finals

The ATP World Tour finals will once again grace London's O2 Arena. Craig Mercer - CameraSport via Getty Image

The last event of the ATP season is here, and as always, the World Tour Finals features a star-studded lineup -- even without the 2016 finalists Andy Murray and Novak Djokovic in the field.

The tourney begins Sunday at 9 a.m. ET on ESPN3 from the O2 Arena in London.

Rafael Nadal is the top seed, although he said Friday at media day he was unsure because of a knee injury that recently forced him out of the Paris Masters.

For now, though, he's still in the field. Here's a look at what we should expect from Rafa and the top eight players with help from ESPN analyst Brad Gilbert.

No. 1 Rafael Nadal (Spain, 31)

2017 win-loss record: 67-11

Titles this year: 6 (inc. French Open, US Open)

Times qualified: 13

Best result: Final (2010, 2013)

An on-going injury to Nadal's right knee has raised doubts about his participation after it forced him to withdraw midway through the Paris Masters, where his form dropped as he played through the pain. The problem could also hinder Nadal if he makes it to London at the end of a spectacular but grueling season. Whatever happens, the year-end world No. 1 is guaranteed after he climbed from ninth place at the start of the season.

Brad Gilbert's analysis: "If he's there, it could be a great celebration of his year. He would love to finally win this event, but the low-bouncing surface is not his favorite, and the questions about his knee won't go away."

No. 2 Roger Federer (Switzerland, 36)

2017 win-loss record: 49-4 Titles this year: 7 (inc. Australian Open, Wimbledon)

Times qualified: 15

Best result: Winner (2003, 2004, 2006, 2007, 2010, 2011)

The crowd favorite should be in good shape after missing the Paris Masters to get ready for London. Federer has been careful in managing his fitness this season, and it has paid off as he has returned to his majestic best, with his last defeat coming back in September against Juan Martin del Potro in the US Open quarterfinals.

Gilbert's analysis: "He's had an amazing year, and you could argue it is right up there as his greatest with 2004, when he won all the majors -- except Roland Garros -- and eight other titles. He's 10-1 against the top 10 this season, and this event really suits his game."

No. 3 Alexander Zverev (Germany, 20)

2017 win-loss record: 54-20

Titles this year: 5

Times qualified: Debut

An up-and-coming star who has beaten Federer and Djokovic in various finals this year, Zverev made it to the last 16 at Wimbledon and will be no pushover. But the German isn't afraid to show his emotions on court and can be erratic.

Gilbert's analysis: "This is an important tournament for Zverev to see where he is at. He's made a tremendous leap this year and has an all-court game. I wouldn't be surprised to see him make a deep run. First time playing in London, he'll be keen to make an impact."

No. 4 Dominic Thiem (Austria, 24)

2017 win-loss record: 48-25

Titles this year: 1

Times qualified: 2

Best result: Round-robin stage (2016)

Thiem arrives in London having told ESPN he's suffering a crisis of confidence after a post-US Open run that has seen him win just two matches. His coach, Gunter Bresnik, says he needs to work on his concentration and stick to his game plan more.

Gilbert's analysis: "Thiem's not as comfortable yet on hard courts and indoors as he is on clay, but he's still dangerous. He's got a big serve, and I'm not sure anybody hits the ball harder than him with their groundstrokes. He needs to work on his return a bit and obviously needs something in London to improve [his slow ending to the season].

No. 5 Marin Cilic (Serbia, 29)

2017 win-loss record: 44-19

Titles this year: 1

Times qualified: 3

Best result: Round-robin stage (2014, 2016)

It's a chance to create a positive memory in the city that delivered a painful, emotional final defeat by Federer at Wimbledon this year. Cilic crumbled under the pressure on Centre Court. It was a rare sad moment in a good all-around season. He made the semifinals of his past three tournaments before the Paris Masters and would expect to get beyond the round-robin stage for the first time.

Gilbert's analysis: "He's a crafty player with great movement considering he is 6-foot-6. He's good on a lot of surfaces and is playing well. He has a chance to secure the highest year-end ranking of his career and could go deep into the tournament."

No. 6 Grigor Dimitrov (Bulgaria, 26)

2017 win-loss record: 44-19

Titles this year: 3

Times qualified: Debut

Dimitrov returned to the top 10 in July after more than two years. He withdrew from the Vienna Open last month citing fatigue after three late-stage tournament runs in three weeks. And he wasn't at his best at the Paris Masters last week.

Gilbert's analysis: "It's good to have him in the mix among the debutants. Dimitrov showed his talent by reaching the semifinals in the Australian Open at the start of the year, and he turned his game around from 18 months ago. We're starting to see now what we expected to get from him three or four years ago."

No. 7 David Goffin (Belgium, 26)

2017 win-loss record: 54-22

Titles this year: 2

Times qualified: 2

Best result: Round-robin stage (2016)

Goffin had some ankle injury problems earlier in the year but won recent titles in Shenzhen and Tokyo. He also made the semifinals in Basel before being beaten by Federer. Goffin, who is blazing a trail for Belgian players, also recorded the biggest win of his career against Djokovic this season.

Gilbert's analysis: "He's not very big but is a deceptively good player. Goffin sticks very close to the baseline and is a good ball-striker. He's probably better on faster courts, when he can take the ball early, so London could be a nice environment for him. With the Davis Cup final against France to follow, he's got a great chance to end the year strong."

No. 8 Jack Sock (USA, 25)

2017 win-loss record: 36-19

Titles this year: 3

Times qualified: Debut

Sock did not know he had a chance to make the London finals when he arrived in Paris. He then looked like he was going out early when Kyle Edmund was beating him at 5-1 in the final set in the second round. Winning the tournament was a breakthrough in several ways: Sock became the first American to win a Masters title since 2010 and the first to make the Tour Finals since 2011. He won three titles in a year for the first time and broke into the world's top 10.

Gilbert's analysis: "Sock had a great few months through Miami but struggled a bit after that until Paris. Making London has been a crazy leap for him, and we've been waiting for something like this. He'll be playing beyond house money after that Edmund turnaround and taking the title. What has happened to him could be career-changing."