One of the four biggest tournaments in professional tennis is well underway in Melbourne, where the top performers vie for the Australian Open grand slam title.

As play progresses into the semifinals, we’re seeing much of what we expected when the tournament started: Novak Djokovic has lost only one set, Roger Federer and Rafa Nadal are once again in the semis, Maria Sharapova is nearing a comeback victory and Kim Clijsters is fighting her little heart out.

But now, in the semifinals, is when every player has something to fight for.

Nadal, coming back almost injury-free, will set out to prove that he still has it in him — that his dominance is not only limited to the French Open and is still a No. 1 contender. His goal is to put together a season that will boost him past Bjorn Borg and Rod Laver, placing him third of the all-time grand slam winners. The Australian Open is the necessary starting point.

Federer is setting off once again to prove he is not washed-up and can close out his historical career with one more grand slam. He’s up to 1,000 career matches at age 30, but that just might be his limit.

Andy Murray will continue trying to win his first grand slam, inevitably failing and solidifying his status as the best-worst professional tennis player.

Djokovic, perhaps, has the greatest challenge of all. He will set out to make history as the greatest male tennis player to ever play the game. He has the chance to become the first male to put together a calendar year Golden Grand Slam, joining Steffi Graf, the only player to ever do so.

The calendar year Golden Grand Slam requires a player to win all four major grand slams, as well as the Olympic gold medal within one calendar year, feats that Djokovic has claimed are his goals. Last time he had a goal, he ended up winning 43 straight matches and became the number one male tennis player in the world.

On the women’s side, the No. 1 spot has a guaranteed opening with Caroline Wozniacki out due to a straight-set pummeling by the injured Clijsters in the quarterfinals. Victoria Azarenka, Petra Kvitova and Sharapova all have a chance at jumping to the top.

Yet the championship is in the sights of Clijsters more than in all of the others. She entered the Aussie Open as the 11th seed, played through an ankle injury, hip spasm and unspecified neck problem in a dramatic three-set victory over Li Na before destroying the No. 1 seed. She is looking to rebound after an injury ridden 2011 and defend her Australian Open title, her last success before an abdominal problem plagued the rest of her season.

So now we find ourselves in the midst of the semifinals with exactly who we expected — minus, perhaps, Clijsters. The question remains, however, who will win it all?

On the men’s side, I think it will come down to the top two: Nadal and Djokovic. Murray is a great tennis player, but he is not the best and cannot beat the best. He lacks the strategic play needed to oust Djokovic and, although versatile, can only play the defensive against Djokovic’s strong groundstrokes and ability to move the ball anywhere on the court.

Federer is undoubtedly one of the greatest tennis players ever to play the game, but his age and match count suggest an inability to top the younger and more aggressive Rafa. Federer has a convincingly failed record against Nadal and his aging is not going to change that.

Even though the championship may be a potential must-win for Nadal if he wants to maintain his dwindling dominance, Djokovic is simply too good to lose.

Both have a similarly heavy topspin on their shots, excel in ball placement and are fast as hell. The only area in which they really differ is the serve. Djokovic has an incredible serve, capable of throwing his opponent past the sideline so he can place it at the farthest possible point on the opposite side of the court. Nadal, on the other hand, has a good first serve, but an easily attackable second serve. When given that opportunity, Djokovic will not hesitate to attack.

I have no doubt that Nadal can keep up with Djokovic in a rally and can probably beat him in an intensity contest. It will come down to the serve and how Rafa responds to Djokovic’s strong baseline play, and I think Djokovic might just be too great of a tennis player, at least off of the clay court.

On the women’s side, the field is more open. I think all of the four contenders are equally skilled, but there may be one area that will determine the winner.

Sharapova may be on her way to a comeback, but I don’t think it will be in Melbourne. Kvitova is strong and getting stronger, while Sharapova is somewhere between rebounding and losing her edge. Kvitova is my guess to be the heir to Wozniacki, most likely earning a place in the Aussie Open final matchup.

On the other side is Clijsters versus the No. 3 seed, Azarenka. Azarenka has been cruising through, yet she proved that she can come out weak when she dropped the first set to the No. 8 seed Agnieszka Radwanska. This brief period of weakness may be enough for the strong-willed Clijsters to dominate. And dominate I think she will do.

I see a Clijsters versus Kvitova final. As much as I will be rooting for Clijsters to make her “Kimback,” I think Kvitova has the weapon to beat Clijsters. Clijsters’s only real weakness is that she has a hard time handling a high bouncer or a hard serve on her backhand side. Few female tennis players have the ability to take advantage of this and that is why Clijsters has been able to excel.

Kvitova, however, has a hard left handed shot that tends to draw her opponents away on their backhand side. Clijsters may not be able to handle this, especially with her collection of injuries. She will play a strong match, but may come up just short to the increasingly powerful Kvitova.

Either way, this Australian Open is one for the books. There is a lot for each player to fight for, and a fight we will see.