For the first time since 2001 he ended the season outside the top 10, and a grand slam failed to include the Swiss' name in the main draw for the first time since 1999.
Federer, in another first, underwent surgery and endured his longest injury break, not contesting a match since re-injuring his left knee in a tough Wimbledon defeat
to Milos Raonic in July.
But on the eve of the new season, the 35-year-old is adamant his time away from the game could prolong his career. He is fit again and convinced he can capture an 18th major and first since 2012 at the All England Club, where the defeat to Raonic was a rare blemish for Federer on the grass southwest of London.
"I do believe it could be very beneficial for the future of my tennis career that I've had this six month layoff," Federer told reporters in a conference call Friday from his training base in Dubai.
"I feel rejuvenated, refreshed. Maybe mentally I needed this rest more than I thought I would. Maybe also my body needed a rest more than I thought I would.
"It was really about taking a tough decision to say, 'I give myself time, I deserve to give myself time.' I take six months off in a 20-year career, it's okay. If you look at six months off in a 12-month season it sounds like there's no chance I'm ever going to take six months off.
"But I tried to look at the big picture. At least one thing I know now, let's say the knee won't be good, I have no regrets. I feel I did everything this year to get my body back in shape and now only time can tell."
While he never feared for his career, it was a slow, unhurried process.
Federer expressed relief in knowing he wouldn't need a second knee surgery -- the first resulted in early February after he got hurt running a bath for his twin daughters -- but encountered a roadblock prior to the start of the US Open in August.
"The good thing about this layoff was that I didn't have to have surgery after Wimbledon," Federer, now ranked 16th, said. "That for me was No. 1 and crucial. I just needed it to rest really.
"I could play tennis. It's not like I couldn't play. I could play standing, I could play walking. But at the end of the day I didn't really need to do it until around just before the US Open. I played again, very quick. There was a bit of a reaction in the knee and that's when you realize ... we knew it was too early."
Federer, reported the New York Times
, began playing points in early October. He went on vacation at the start of November for almost two weeks, thereafter increasing the intensity of his workouts in Dubai where he has long trained.
There appeared to be no issues for Federer on Thursday, when he streamed a practice session
with rising Frenchman Lucas Pouille live on Periscope. As of Friday morning, the session, or parts of it, had been viewed more than 700,000 times.
'Quality time with the kids'
"The last six weeks for me have been crucial and obviously the last two weeks in November when I came back from vacation were really important for me to see how much load I could put on the leg," Federer said.
"Very quickly we realized at the end of November, early December -- I was not at 100% yet, let's be honest, because you always need more time, more reassurance -- but things went very well. I didn't have any setbacks. In the end it all worked out according to plan."
Off court, the extended spell on the sidelines meant Federer could be less cautious when spending time with his two sets of twins, not having to take into consideration an upcoming match.
"It was great to spend more quality time with the kids, or me with more energy rather than having to worry, 'Careful, I have a match tomorrow' or 'I need to go to bed' or in the morning 'Don't wake me up' kind of thing because daddy needs to sleep a little bit," Federer said.
Given his recovery, those matches are coming, though.
Federer said he played "very well" in more than 15 sets in the past few weeks with Pouille and two other established pros, Borna Coric of Croatia and Damir Dzumhur of Bosnia-Herzegovina.
He might practice once more on December 26 or 27 in Dubai before flying to Australia, where his competitive return takes place at the Hopman Cup in Perth in the first week of January.
Federer said the round-robin format -- he is guaranteed of playing in more than one match -- is, possibly, what he requires.
"Maybe this is the year compared to anytime before that I do need matches," he said. "Matches just have a different intensity and your body reacts according to it."
Is this the year Federer does claim his 18th major? He maintains it's still a possibility, even with the recent dominance of Novak Djokovic and Andy Murray.
"I do hope it's around the corner," Federer said. "If it's not, it's not. My mindset is always very positive that it can happen or should happen or that I am doing everything that I can to possibly achieve that.
"I think I've been awfully close the last few years but that doesn't count because it wasn't winning.
"Time will tell if it's going to be possible or not."