There was a time when betting on Novak Djokovic with your life might've been a legitimately good idea. It wasn't that long ago, either. In the heady days of 2015-2016, Novak was just better than every tennis player on the planet on every surface. In a one year stretch, Djokovic won all four Slams, 5 Masters titles, World Tour Finals and two ATP titles he appears to have won for the fun of it. It was absurd.Yesterday, he lost to Martin Klizan in the first round of a 500 event, his fifth loss in seven matches. If he doesn't back up his points from Madrid, Rome and Roland Garros, he could fall into the 20s or lower. Novak Djokovic put together the highest peak imaginable in professional tennis. His fall has been obscured by the re-rise of Nadal and Federer, more marketable personalities who have stolen the show. For me, the end of this decade will always be marked as the Djokovic decline phase.I don't focus on the achievements from that stretch anymore. I focus on the level of tennis that has dissipated. Let's look at his 6-2, 6-0 rout of Milos Raonic [note]the 2016, not injured Milos[/note] in Indian Wells from 2016.https://www.youtube.com/watch?v=7GYIQTUxF9AMilos Raonic was probably the second or third-best server in the world. HE LOST 6-2, 6-0! HE ONLY WON 43.3 PERCENT OF HIS SERVICE POINTS! In 2015, Djokovic's numbers made him simultaneously one of the best servers on tour (89.5 percent hold percentage. By comparison, Federer is at 89.2 over his renaissance period in 2017-18.) and possibly the greatest returner of all time. Because Federer and Nadal are intent on dominating tennis through 2020, I fear that Djokovic's peak will get erased from the collective memory. Of course, the list of insane Djokovic matches extends well into his breakthrough as world No. 1 after his win at Wimbledon 2011. It's simply astounding.While watching the Sarasota Challenger on a free stream, I heard an amazing statement as Taro Daniel battled through a third set against No. 241 Guilherme Clezar."Daniel's going to have to summon the same level he needed to beat Djokovic," Mike Cation, the erstwhile voice of the USTA Pro Circuit, said.It seemed completely impossible for Taro Daniel, currently in the process of getting blown off the court by world No. 241 Guilherme Clezar, to have ever challenged Novak Djokovic, let alone won a match. Taro Daniel has no weapons except a rapier-like backhand that only works when he has the confidence to hit with depth. His serve is mediocre, his net game is bad and he pushes like his life depends on it. He also apparently has a level that can now defeat Novak Djokovic.Novak is bad now, unquestionably. There are few top 100 players who can boast such an impressive string of horrendous losses in their career. Look, Djokovic got completely crushed by Benoit freakin' Paire. I get that he's getting back to health and he was injured, but come on. Since when does Benoit Paire routine anybody??!What's wrong with Djokovic? For everything that's been said about his personal life, his coaching choices, his fitness and his motivation, the metrics bear out a simple answer. His serve is terrible now. His ace percentage in 2018 is 3.8 percent, which is a massive fall from his career average of 6.9 percent. Watching his match against Klizan, it seemed like Marty could basically return wherever he wanted. Djokovic could not get control of any points in the first or third sets. He is at the mercy of every halfway decent returner in the game. Without his elite serve, he is liable to fall behind early in any match. Compared to his level last year, when he was a solid top-ten player who could reasonably expect to make the semis in most tournaments, Djokovic has fallen into a slump reserved for the likes of Guido Pella, Steve Johnson and Robin Haase.With the pain in his elbow stopping him from keeping his old service motion, Djokovic has opted for a more compact service motion. In practice, the serve has been ineffective, mostly because it's easily readable. Djokovic isn't fooling anyone. After watching Klizan, Thiem, Chung and Paire just tee off on nearly every Djokovic serve, it's clear that experienced players just know where it's going. You wonder whether he can't hit serves out wide down the T confidently or whether he's just lazy.Without any semblance of his old unbreakability, Djokovic has relied on his defense to keep him in matches. However, he's almost 31 now, and his once elastic movement is not quite what it used to be. He looks slow and lethargic. Even though we are comparing him to the absurd speed he showed in 2015 rather than the tour average, he still looks a step behind. Djokovic has never needed to smack the crap out of the ball to be effective. In fact, during his stunning 2015-16 seasons, he was usually out-grinding and out-thinking opponents rather than overpowering them. There are still times when his speed and tennis brain keeps him in matches (see the first set against Thiem). When he can get ahead of players and get on a roll, he can still produce some good tennis. But it's not enough anymore.In many ways, Djokovic is lucky he hasn't gotten to play Nadal, Federer, Del Potro or even someone like Dimitrov or Cilic in the last few months. Any top five player can boss Djokovic around the court with relative ease now. If Martin Klizan, returning from his own injury problems, can do it now, then anyone can. Those days are coming. Once his points fall off and Djokovic must battle through draws, who knows what'll happen.Again, this is just absolutely stunning. I never believed that someone as good at everything tennis-related as Djokovic could just be...bad. If anyone would have a base level of competency in the ATP, it should be Novak. There are plenty of pushers and players with no power who routinely hang around by grinding out matches and getting hot at the right time (see Mannarino, Adrian or Dzumhur, Damir). With the tour being so weak right now, it just seemed impossible that Djokovic could just continue to be this awful. But he is.Instead of wondering why or trying to project his triumphant return, I think we should simply gawk at how precipitous this decline has become. Imagine if LeBron James entered the 2018-19 NBA season and was just a barely replacement level basketball player. Imagine if Tom Brady comes back next year and is worse than Blaine Gabbert. What if Mike Trout suddenly become terrible for two months. This should not happen to once-in-a-generation talents. It's impossible to believe, and yet as Djokovic's losses pile up, it's becoming reality.