By Martyn Herman
MILAN (Reuters) - The huge hoardings around the O2 Arena promoting the season-ending ATP Finals will get some quizzical looks this year with several familiar faces conspicuous by their absence.
Home favorite Andy Murray, who memorably won the 2016 title to end the year ranked number one, three-times London champion Novak Djokovic, who Murray beat, pugnacious Swiss Stan Wawrinka and Kei Nishikori, along with his Japanese fan club, are all absent after succumbing to injuries.
Nadal's participation is also clouded by a knee injury which hardly augurs well for the 31-year-old claiming the only big prize to elude him. He practiced on Friday but was less than positive about his chances of being 100 percent fit.
Should Nadal pull out, and the organizers will be crossing their fingers that he doesn't, the path would seemingly be clear for Federer, although anyone assuming the Swiss will claim the title for a seventh time is discounting the dangers that lurk.
Zverev, the 20-year-old German, has won five titles in a stunning season and catapulted to third in the world rankings.
The huge 02 Arena is the perfect stage for the man expected to be challenging for grand slams next season.
Dimitrov, long-compared to Federer in style, is finally living up to his potential while Goffin, all effortless movement and silky strokes, is a purists' dream.
Sock is the one real outrider. He was 24th in the Race to London entering the Paris Masters and even had a golf trip booked in at Augusta National during the ATP Finals.
His first Masters 1000 title meant a hasty change of plans as he grabbed the final spot to become the first American to qualify since Mardy Fish in 2011.
Croatia's Marin Cilic, the only other grand slam champion in the draw apart from the Federer and Nadal, is back in London for the first time since his tearful Wimbledon final loss, while unsung Austrian Dominic Thiem makes up the cast.
But there is no doubt who has the lead roles.
A year ago Federer and Nadal were both missing from London and many thought they might never be back with rackets in hand.
Yet between them they have returned from injuries and defied Father Time by dominating the year -- Federer winning the Australian Open and Wimbledon to reach 19 majors, with Nadal taking a 10th French Open and the U.S. Open.
Should Nadal declare himself ready and able he will face Goffin in the Pete Sampras group on Monday evening.
Dimitrov and Thiem are also in Nadal's group while Federer kicks off against Sock on Sunday before Zverev makes his eagerly-awaited debut against Cilic.
(Reporting by Martyn Herman, Editing by William Maclean)