Not much is my quick answer.
We already know that Novak Djokovic is not playing at the airtight level he played at from January 2015 through June 2016. We already know that Roger Federer is 35 and will not be as consistent in his results as he was at 25. Consider that Nick Kyrgios hit 25 aces versus Djokovic. Kyrgios hit enough aces to win 6.25 games in terms of raw points. That doesn’t even include service winners. In a 2 out of 3 set match, hitting 25 aces is a recipe for a lot of success. Roger Federer failed to close out a winnable situation. He is in his second tournament back from a six month layoff. Lack of match play caught up with him. In a 3 of 5 set format, he was able to play through some rust that a 2 of 3 set format does not permit.
Where do they Stand?
Novak Djokovic is a player who is looking for his better form. That is obvious. He is physically capable of returning to that form as far as any of us know. Mentally, he may be a bit burned out after climbing so high. Physically, something may be ailing him (shoulder? elbow?). However, he doesn’t appear to have a long-term injury. Previous champions have sometimes had lulls after a period of dominance and returned to win big prizes (Federer and Nadal have each done this), but some previous champions don’t ever return to places near the spot they once occupied (Wilander and McEnroe come to mind). Boris Becker may be a good predictor for his former charge. Becker had a great 1989 and 1991, but then went into the wilderness. That did not prevent Becker from rebounding in 1995 and 1996. I think Djokovic is capable of reclaiming something close to his top form, but I am not sure when to expect that. I doubt it will take him 4 years to recover his form though.
Roger Federer is 35 and has a surgically repaired meniscus. I am sure his surgeons and team are as good as it gets, but anyone expecting Roger to win 11 or 12 titles in 2017 as he did each year from 2004-2006 is not seeing clearly. Roger is talented, has a lot of weapons, plays aggressive tennis so the match is often on his racquet. That gives him a great chance to win in many matches. Those chances are augmented by a 3 of 5 set format with days off between matches. The Swiss can play through bad patches and recover from matches at Grand Slam events. I think he is obviously a threat as he proved in Melbourne and even at Wimbledon and the Australian Open last year, but he is not going to overwhelm the tour on a weekly basis.
Who saw that coming?
Roger Federer suffered a shock defeat in the Dubai Championships second round.https://t.co/GpQJX3qsg3 pic.twitter.com/wNR3laa2eP
— BBC Sport (@BBCSport) March 2, 2017
Tennis Abides: Dan Martin on Tennis and Life 