How much longer will Serena, Novak hold on to No. 1 ranking?

With the clay season just beginning, what better time to take stock of the world No. 1s, Serena Williams and Novak Djokovic, who likely won't have easy rides through perhaps the most taxing part of the year.

On Wednesday, Djokovic suffered a stunning loss to Jiri Vesely in the opening round of the Monte Carlo Masters. Djokovic was broken three times in the 2-hour, 6-minute affair, handing him his first loss (excluding his withdrawal in Dubai with an eye injury) of the season.

This sure isn't the way he wanted to start the clay season. Still, the larger picture suggests Djokovic is the player to beat moving forward, unlike his WTA counterpart, Serena, who has not found a consistent groove all season.

With that, a look at where the respective top players in the world stand:

Serena Williams

She hasn't won a title since last August. At age 34, I have my doubts whether she has another comeback left in her, which leads to this question: Will she be the No. 1-ranked player at year's end?

If the early 2016 trends continue, then the answer is obviously no. For the first time in a number of years, we have a legitimate contender ready to overtake Serena. Victoria Azarenka enters the clay season coming off back-to-back titles in Indian Wells and Miami. She is motivated, fit and well-coached. Wim Fissette and Sascha Bajin take detailed notes -- everything from patterns of play to technique and footwork to Azarenka's emotions -- every time Azarenka takes the court. Afterward, her team reviews these reports with great detail.

Azarenka's transition game and forward-thinking approach is the best in women's tennis right now. She has been brave with her second serves, even if it has meant double-faulting more often. I like that mentality. Poor second serves that get crushed by great returners is demoralizing. At least an aggressive second serve is something you control.

Azarenka plays with a strong intensity and maturity -- but with a worldly perspective that was not there before. After missing so much time with injuries, Azarenka realizes her time to take control of the game is finite, and she's taking advantage of this precious period.

As for Serena, she remains a staunch competitor, but we've seen her fold during the salient moments for quite some time now. Is it a loss of focus? Is the pressure of catching Steffi Graf's mark off 22 majors -- an Open era record -- weighing on the American's mind? As a professional player, these milestones can be distracting with so many eyeballs vetting every move.

And when you consider the points Serena has to defend now through September -- and the lack of points Azarenka has to defend -- it seems more than likely the world No. 1 ranking will change hands by the end of the year.

Novak Djokovic

So much for Djokovic extending his all-time Masters 1,000 lead this week in Monte Carlo. The defending champ is out in his opening match. This is a huge shock considering he swept Indian Wells and Miami for the third straight year -- one of the most impressive feats in tennis.

Despite the loss, Djokovic is, of course, the best player in the game. He had some tense moments during the spring hard-court swing, but I continue to be impressed by his mindset to always want to improve, even if it is just making subtle changes. Adding Boris Becker to his coaching team was a stroke of genius. He has helped Djokovic's second serve, which might be the best in the game. Djokovic is far more comfortable moving to net and ending points quickly.

Becker understands as well as anyone how to manage, both physically and mentally, great rivalries. He's passed this on to Djokovic, who has owned one rival in Rafael Nadal by winning six straight matches, and who has beaten Roger Federer in nearly every meaningful match in the past couple of seasons.

Many remember when Becker first joined Djokovic's team and scratched their heads. In a key moment at the Australian Open two years ago, Djokovic served and volleyed in a tight moment. What was he thinking? This isn't his game. What is Becker doing to this guy? I was among those who doubted Djokovic's net prowess. But like Djokovic has done throughout his career, he continues to prove people wrong.

Djokovic is also well-balanced off the court; he often speaks of his Zen-like disposition. In a game in which small nuances make a significant difference, Djokovic is far ahead of the field in this component of the game. Even when he's not playing well, as we saw in his 100-unforced-error performance against Gilles Simon at the Aussie a couple of months ago, he believes he will win -- and does.

But the clay season is here, and Djokovic is off to a rough start. He has won a lot of events on dirt, but not the one he desperately wants -- the French Open. This year, I have to still think it's his for the taking, even after a rare slip-up Wednesday. He's simply the best ball striker in the game, and with Federer and Nadal entering this arduous stretch with question marks, who is going to stop the Serb? Sure, a Stan Wawrinka can catch fire (as he did last year), or an up-and-coming player like Dominic Thiem could foil some dreams or an aberration like Vesely could happen, but would you bet on that happening? I'm not sure I would.

This is the Djokovic era, and if he can extend it another three or four years, with at least one Roland Garros title, then he could be considered the greatest ever.