'I'm confident that if he does feel 100 per cent it's not going to be a problem for him to play at the very top. "You don't get any presents when you're playing against the Top 8".
The question has been hanging around like a bad smell since they announced the lucrative one-off contest last month, which would have paid them at least $1 million (£770,000) each. Reaching his maiden Grand Slam final at the French Open this year, losing to Rafael Nadal, he comes into this event with a 2-4 win-loss record in London.
That he achieved this remarkable return to form alongside his old coach, Marian Vajda, with whom he reunited this spring, made the choice of Coach of the Year a straightforward one, too: The Serb was picked by his fellow coaches to receive the honour.
What was to follow this year has seen Nishikori dig deep through perseverance, not giving up his childhood dream of becoming a professional tennis player and pure determination to reach the top of the game again. Federer in straight sets. At Cincinnati, Federer hadn't lost in seven final appearances and hadn't suffered a service break since 2013.
The victor of this year's Nitto ATP Finals could receive almost $3 million in prize money.
Does all this mean that the other six men in the fray at the Finals will be reduced to supporting actors or is there a pleasant surprise in store like Grigor Dimitrov's triumph a year ago? Kevin Anderson and John Isner, who have qualified on the back of their career-best years, are long shots.
Alexander Zverev and Dominic Thiem, from a generation further below, are still developing. Bulgarian Dimitrov, who climbed to a career-high No. 3 after winning in London a year ago, hasn't even made it, slipping to No. 19 in the world.
The Swiss is a six-time champion at the year-end championships and is the No. 2 seed.
Fast forward to past year and after cutting his season short in August after scans revealed a tear in one of his tendons, Nishikori was forced to ask himself the question that no top sports athlete in their prime ever was to contemplate - retirement. There were Lleyton Hewitt, Andy Roddick, Juan Carlos Ferrero and Marat Safin.
Not that Federer will go home empty handed.
Where can I stream Roger Federer vs Kei Nishikori? . He did beat Sampras at Wimbledon early on but he didn't really win as early as a lot of great champions did. "I've learned a lot in each of the past two years and it's such a great event, with so many spectators and every session is sold out, which is absolutely unbelievable", said Thiem.