The 19-time Grand Slam champion says some of his fellow players struggle with their media obligations, fearful of being misinterpreted.
The 12-times Grand Slam champion, Djokovic, seeded an unfamiliar 14, was too strong for American Donald Young, winning 6-1, 6-2, 6-4.
He said he had met Federer several times and would like to see more of him, "but obviously his time is pretty well taken care of".
Federer, who had an incredible 2017 after returning from injury, winning not only a fifth Australian Open but a record eighth Wimbledon, opens his title defence against Slovenia's 51st ranked Aljaz Bedene on Tuesday.
Djokovic made a winning return on Wednesday after six months out with an elbow injury, declaring he felt "great" after beating world number five Dominic Thiem.
Asian number one Nishikori has not played competitively since last August after suffering a torn tendon in his right wrist during a practice session at Cincinnati.
From 2019, Wimbledon and the Australian, French and US Opens will have only 16 seeded players in the men's and women's draws.
David Goffin, who had never beaten his much-vaunted opponent, looked aggressive and did not allow Roger Federer to dictate play as the Swiss player lost his precision.
Roger Federer has never lost a single match to any of the remaining players in the ATP World Tour Finals 2017.
David Goffin had a shaky start, losing the first three games, but lost only two games after that in the entire match as he romped to victory in 71 minutes.
Jack Sock is the first American to reach the semi-finals of the ATP World Tour Finals event since Andy Roddick in 2007.
By extending his current winning streak to 13 matches, the 36-year-old advanced from the Tour Finals group stage with an unblemished 3-0 record for the 10th time in his glittering career.
The world number two saw off a spirited challenge from the much-hyped German 7-6 (8/6), 5-7, 6-1 in front of a boisterous pro-Federer crowd to become the first man through to the last four at the O2 Arena.
As the world's most formidable player on clay, Nadal has in the past suggested it would be better if the season-ending Tour Finals at least occasionally took place on his preferred surface.
Roger Federer hasn't triumphed at the season-ending Tour Finals since 2011.
Rafael Nadal and Roger Federer get star billing in London after each won two Grand Slams and locked down the top two rankings spots as Novak Djokovic and Andy Murray had injury-plagued seasons to forget.
Rafael Nadal reclaimed the top ranking in August for the first time in three years.