With Federer certain, young Austrian Dominic Thiem is third in the ATP race to London, followed by Stan Wawrinka, Alexander Zverev, Djokovic, world number one Andy Murray, and Belgium's David Goffin.
Rafael Nadal has become the first player to book his place at the season-ending ATP World Tour Finals in London in November after the Spaniard won a record 10th French Open title in Paris on Sunday.
Nadal has claimed four titles on the tour this year, winning 46 matches and losing just six in a stellar 2017.
"I've had a great season so far and I am happy to have already qualified for London," the Spaniard told the ATP website.
"I could not play last year because of injury so I look forward to returning in November."
Nadal has never won the event, but finished runner-up in 2010 and 2013, losing out to Roger Federer and Novak Djokovic, respectively.
The ATP World Tour Finals, which features the world's top eight singles players and top eight doubles pairs, will be held at London's O2 Arena from Nov. 12-19.
Federer is next in line to qualify for the event, for which he is a near certainty after winning the Australian Open earlier this year.
The Swiss will return to action in Stuttgart this week after sitting out the entire claycourt season to prepare for Wimbledon.
Young Austrian Dominic Thiem is third in the ATP race to London, followed by Stan Wawrinka, Alexander Zverev, Djokovic, world number one Andy Murray, and Belgium's David Goffin.
Players earn their place at the season finale by finishing in the top eight of the race to London standings on November 6, when the World Tour regular season concludes after the Paris Masters.
The race is a calendar-year points race that starts at the beginning of each season in the first week of January.