The professional tennis season has been over for the better part of a month now, and what a season it was. It started out predictably, was hit by a few major surprises and ultimately ended in a way that few envisioned. Here are the major highlights of the 2016 professional tennis season, as well as a few predictions of what we can expect on the pro circuit next year.

1. Djokovic holds all four Grand Slams.

Something that not even tennis greats Roger Federer or Rafael Nadal had ever achieved occurred this year: Novak Djokovic won four Grand Slams in a row, beginning with Wimbledon in the summer of 2015 and culminating with winning the French Open this past summer. One can count on one hand the number of players who have achieved such a feat (the last time it happened was decades ago), and tennis fans all over the world were fortunate enough to witness this rare event.

2. Djokovic loses early at Wimbledon.

Unlikely candidate American Sam Query provided the upset of the year by bringing former world number one Novak Djokovic back down to Earth by beating him in the third round of Wimbledon a mere two weeks after his triumph at the French Open. This defeat would set the tone for the rest of Djokovic's year.

3. Sharapova banned after being convicted of using illegal medication on court.

Controversy surrounded the decision to ban tennis star Maria Sharapova after it was revealed that she took a substance deemed illegal by tennis authorities during the Australian Open. Maria's main defense was that the drug was only made illegal this year, and she had been using it for nearly her entire career with no issues from officials. In any case, Maria will be making a return to the tennis scene in 2017 once her ban is officially lifted.

4. Federer has his iffiest year to date.

Swiss legend Roger Federer began his year on a rough note by requiring knee surgery, which kept him off of the tennis scene until May. Still not feeling ready to compete, Federer made the decision to withdraw from the French Open, ending his streak of 65 consecutive Grand Slams played (a record). After reaching the semifinals of Wimbledon, Federer once again had to take time off of the court, this time for the remainder of the year (missing the Olympics as a result, which Federer had previously cited as the most important event of the year for him).

5. Nadal withdraws from the French Open.

The Mallorca native had only lost at the French Open twice (a record frequently cited as among the best in the history of the sport), which made the decision to pull out of the tournament due to injury a difficult one. The rest of Nadal's year was spotty, although he saw success by reaching the semifinals of the Olympics in singles and winning gold in doubles. However, Nadal lost early at the US Open, and thereafter ended his season prematurely to begin preparations for the 2017 season. Both Nadal and Federer are at the tail ends of their careers, and for both of them to play so little tennis in a season is a testament to that fact.

6. Andy Murray beats del Potro in the Olympic Gold Medal match.

Del Potro kicked his Olympic quest for gold in the most outstanding fashion, beating former world number one Novak Djokovic in the first round. The Argentine was coming back from a lengthy time away from the court after enduring multiple surgeries on his wrist, and to the surprise of all began dominating the entire field, ultimately beating Rafael Nadal in a thrilling semifinal match to face off with world number two Andy Murray. Murray was too good, however, and made history by becoming the first man to win two gold medals in the men's singles event twice in a row.

7. Wawrinka wins the US Open.

Stan Wawrinka upset Novak Djokovic in the final of the US Open, bringing his total number of Grand Slams won to three. A top-twenty player for most of his career, Wawrinka has proved in the last few years that it isn't too late to improve, and is now ranked among the top five players in the world.

8. Murray becomes the world number one after going on 24-match-win streak.

Who saw that one coming? Andy Murray, who has long been in the shadows of Novak Djokovic, Roger Federer and Rafael Nadal, ended up having the year of his life after winning Wimbledon, the Olympics and a multitude of other titles (along with becoming a father for the first time) to take the top spot away from Djokovic and end the year as number one. Fun fact: Murray's brother, Jaime Murray, is also the world number one in doubles, the first time two brothers have held a number one position in tennis at the same time.

9. Kerber becomes the women's number one.

Angelique Kerber had an incredible year to say the least, winning the Australian Open, making the finals of both the Olympics and Wimbledon and winning the US Open. That was enough to steal the number one spot away from Serena Williams and see her begin her 2017 season atop the women's rankings. Whether she can maintain that spot is another question, but seeing as Serena is nearing the end of her career, there is a decent chance that Kerber could see similar success next year.

10. What's next?

2016 was a year full of surprises, both inside and outside of the tennis world. With regards to tennis, however, it is safe to say that Murray and Djokovic will continue their rivalry on top of the game, with Djokovic making a big push to get back the number one spot (he recently split with long-time coach Boris Becker, however, which throws an interesting element into the equation). The returns of Federer and Nadal, as well as Sharapova, will all be carefully scrutinized -- all three are members of a fading generation, so it is likely that 2017 will be the last year for one of them. The new generation of players is fully aware of this, and so the most pressing question in my opinion is whether a member of the "NextGen" will make a serious push towards the top in 2017.

11. What do you think?

It's been a wild year for tennis, and next year is shaping up to be just as interesting. Please comment your thoughts or predictions for the 2017 season below, then check back later to see if they came true or not. See you next year!