(CNN) -- Roger Federer arrives in London for the ATP World Tour Finals with a point to prove.
The 17-time grand slam champion, arguably the greatest player to ever pick up a racket, is outside the top four of the world rankings for the first time in a decade.
Drops in form and fitness have plagued the once all-conquering Swiss, but one prominent coach is confident Federer can return to the peak of the sport if he can address a niggling back problem.
"He's fighting against his back which is hurting him very often during the year," Patrick Mouratoglou, coach of women's tennis powerhouse Serena Williams, told CNN.
"He lost many matches because of the back, he lost confidence because of that and also he cannot practice the way he wants because his back is hurting.
"He's been out of shape too many times in the year and at that level you cannot perform when you are out of shape."
Federer, who recently split from coach Paul Annacone, is appearing in the season-ending showpiece for a record-equaling 12th year in a row.
The Swiss has also won the tournament more times than any other player in history, with Federer heading to London's O2 Arena in search of a record-extending seventh title.
Mouratoglou is confident Federer, currently down in seventh in the world rankings, still has what it takes to challenge the top players in the men's game.
"His back is the main thing," he added. "I cannot say he would be No.1 still because the men are very close to each other.
"I think Rafael Nadal has always caused problems to Roger, but he would be in the top three."
World No. 1 Nadal tops the bill at the World Tour Finals and is bidding for a first win at the event, while six-time grand slam champion Novak Djokovic is looking to add to the titles he clinched in 2008 and 2012.
British fans will be disappointed that Andy Murray misses as he continues his recovery from a back injury, but 2009 U.S. Open champion Juan Martin del Potro will be in attendance.
David Ferrer, Tomas Berdych and Federer's compatriot Stanislas Wawrinka will also take part in the eight-man round robin competition.
Frenchman Richard Gasquet completes the line-up. The world No. 9 has benefited from Murray's withdrawal.