Here is the data, and it shows his fading ability to return compensated for improving service. This is not a troll thread, an attack or an attempt to invalidate any of his accomplishments.
In fact, the way he is losing some of his defensive ability is very slow, very subtle and may be the slowest decline we've ever seen in the history of tennis.
Aside from 2006, when he was just breaking into the ATP, his worst year ever for serving on hard was 2010, where he barely got over 82% of service game for the year. In 2011, his miracle returning year, he was very close to 85%. This year, so far, he is at very close to 91%. That's a 6% net gain on the service game, which is amazing. But his return game percentage is currently at 28%.
28% would be a dream come true for most top players, but for him it's not only a 13% drop from 2011, his miracle returning year, it's 4% below his career average.
He will have to raise that this year to keep the same dominance he's had in the past. Does that mean he's no longer the best hard court player in the world? No, because there is so far no one who is approaching that magic 60% number for all games, so he's still the guy to beat.
But keep in mind that his career average on hard is 59.76% of all games, and that average includes not only 2017, his worst year since 2006, but also every year up to 2010. So when his yearly average fall below his career average, that's a decline. Please be aware that a decline for Djokovic is at the point mostly superior to everyone else out there, so keep it in perspective.
Just remember that anything under 30% of games won on return is well below his career average, a whopping 32%. So look carefully this year to see if that number goes up, or if it goes lower.
Remember: all aging players have a decline in defensive skills. It's just a matter of how fast it happens, and now long it takes before they fall to a point they can no longer compensate.