One day after an emotional win over her elder sister, Serena Williams faces a recurrent thorn in her side as she stands three wins from a second Serena Slam.

Serena Williams vs. Victoria Azarenka

It has been a topsy-turvy Wimbledon for the ladies, but the draw has been whittled down to eight remaining competitors.  Many of them are surprise quarterfinalists, but top seed Serena Williams is right where she is supposed to be.  She has overcome some challenges to reach this point, and on Tuesday, she will face what could prove to be her stiffest challenge yet in opponent Victoria Azarenka.

To the casual tennis observer, it might appear that Azarenka has little chance of causing the upset.  She has only won three of her previous 19 encounters against the American, and her most recent victories came back in 2013.  But regular fans and analysts know that the Belarusian is the closest thing that Williams has had to a rival the last few years.  She has pushed Williams the distance in a number of their past meetings, including the two that occurred earlier this season.  Azarenka actually held match points against Williams in Madrid and came within a couple of points of derailing the American's title run at Roland Garros.  Azarenka will be hoping that the third time is the charm as she attempts to find a way to get the job done against the current No. 1 at Wimbledon.

There is no doubt that Azarenka has a game that can defeat Williams if the American is not at her best.  The former No. 1 gets good pop on her first serve and groundstrokes, and she is also relatively versatile off both wings in terms of her ability to go up the line or find the angles.  In addition to that, she has added more variety to her repertoire.  Azarenka has improved her skills at the net and is able to more consistently utilize the drop shot and backhand slice, which are all tools she will need to employ against Williams to keep the top seed off balance.

Keeping Williams off balance is going to be crucial to Azarenka's odds of success, since the bottom line is that Williams is the better player.  She is the more powerful shot-maker, and she holds a decisive edge in the serving department and with her movement.  As such, she is in a more favorable position to turn defense into offense and dictate play.  Since Williams has the superior set of weapons, the onus will be on Azarenka to red-line her own game in order to take Williams out of her comfort zone.

Knowing what to do against an opponent is just half the battle.  Believing that victory is possible is the other, and that is perhaps where Azarenka has stood out among other competitors when it comes to playing Williams.  Despite her dismal record against the American, there is never a trace of fear or resignation when she takes to the court against the current No. 1.  She believes that her game is good enough to garner the win, and although rare, that has been the case in the past.  In Tuesday's match, Azarenka will come in buoyed by the knowledge that she has been on the cusp of victory against Williams on two occasions already this year, and all of the pressure is squarely on Williams as she attempts to not only secure another Wimbledon title, but continue her quest for the calendar Slam. 

Rest assured that Williams will aim to come out of the gates sharp in this contest, however.  She is aware of how dangerous Azarenka can be.  She also knows that she can ill afford to fall into a hole similar to the one she found herself in against Heather Watson in the third round, since a player as experienced and accomplished as Azarenka is a lot less likely to let her off the hook.  She will not be taking Azarenka lightly and will undoubtedly look to get down to business straight away.

It is safe to assume that nobody will be taking this match lightly.  It is the undisputed blockbuster of the four quarterfinals, and while the odds favor Williams advancing to the semifinals, if this match goes the way of their two previous encounters this past spring, fans are in for a real treat.

Agnieszka Radwanska vs. Madison Keys

One of the hottest topics at this year's Championships has been the way the bottom half of the women's draw has opened up with so many of the top seeds tumbling out early.  But the misfortune of those favorites has left the door wide open for others, and Agnieszka Radwanska and Madison Keys are two of the women keen to take advantage of the opportunity that lies before them.  They will square off against each other on Tuesday with a place in the final four of Wimbledon hanging in the balance.

Few likely had Keys or Radwanska reaching the quarterfinals of Wimbledon, let alone facing each other at this stage of the tournament.  It is not just because there were higher seeds who were supposed to reach this point in their stead, either.  Instead, it was that neither woman honestly looked poised to make such a run. Keys shocked everyone when she reached her first Grand Slam semifinal at the Australian Open earlier this year, and she accumulated some impressive wins on that journey.  Unfortunately for her, outside of her finalist appearance in Charleston, she has struggled to live up to the new expectations that came with that breakthrough in Melbourne. 

Radwanska has arguably had it even worse than the American.  The Pole was once ranked as high as No. 2, but she has failed to find her game when she needed it most throughout the bulk of this season.  As a result, she has seen her ranking slide outside the top 10, and though there were signs that she was turning things around with her semifinal showing in Nottingham and finalist appearance at Eastbourne, many were skeptical that she had sufficiently recovered enough from her slump to cause any real damage at Wimbledon.

Keys and Radwanska have surprised many with what they have accomplished thus far on the lawns of the All England Lawn Tennis Club, and for one of them, the dream will continue to the semifinals.  Each has a decent look at advancing.  Keys, for example, has a lot of obvious weapons.  Not only does she strike the ball bigger than Radwanska, but she is right up there with the likes of the Williams sisters and Petra Kvitova in terms of raw power.  She possesses that strength off both wings and her serve, and if she is at her best, she could potentially take the match right out of the Pole's hands.

While Keys is better equipped to take control of the match, however, that does not mean that Radwanska is defenseless.  On the contrary, she has some tricks up her sleeve that are highly useful on these hallowed lawns.  She can execute a low slice with both her forehand and backhand and put all manner of spin on the ball to keep her opponent off balance.  Furthermore, she is the quicker of the pair and does an exceptional job redirecting pace.  In addition to that, she has been very effective at getting to the net the last few weeks, and if she can do that with any regularity against Keys, she will be able to rob the American of time to set up for her own powerful shots.

Radwanska has a mental advantage in this match as well, in that she has beaten Keys in all three of their prior meetings, two of which were straight-sets victories.  She is also a former Wimbledon finalist and has more experience playing these types of matches.  But Keys will not be lacking in belief.  She knows she can go at least one round better, since she has already done so with her run in Melbourne this past January.  Additionally, the one match in which she pushed the Pole the distance came at Wimbledon in 2013, and she knows flipping that script is not out of the realm of possibility in 2015.

This is a fairly even match, and only time will tell which of these women will come out the victor.  It could be Keys, who like a hammer batters Radwanska into submission.  It could be Radwanska, who like a scalpel carefully slices and dices her way to victory.  Either way, it has been an excellent Wimbledon for both competitors.