After the 2013 season finished, Djokovic only won 6 slams out of 12 GS finals. Meanwhile Federer alrady had 17 slams, Nadal finished the season strong with 13 slams. What made it even more devastating: He lost a winnable 2013 RG match against Nadal, was straight-setted by Murray in the Wimbledon final and lost again against Nadal in the USO final. Remember the net at RG? The point the turned around the match?
By the end of 2013 I accepted the fact that he will probably max out at ~10 slams and Federer/Nadal will both come close to ~20. That Federer and Nadal will become the 2 greatest tennis players of all time and Djokovic just an ATG comparable to McEnroe, Lendl and Agassi.
Then the 2014 season started where he lost to Wawrinka in the AO QF and again to Nadal at RG. People were already talking that he is done winning slams and I slowly started to believe that too. He was still at only 6 slams and Nadal got to his 14th. The gap just wasn't closing, it widened even further.
Wimbledon 2014 changed everything, really everything. It was only his 2nd title at this event and who could have imagined that he would be at 7 Wimbledon titles 8 years later? Not only that, be he transformed into a GS final winning machine. He lost 7 out of his first 13 GS finals and then won 16 out of 20! 10 years ago he was barely in the Top10 of ATG and now he is considered by many to be the GOAT of tennis.
What a crazy ride.