Maria Sharapova against Germany.
On Friday, the Russian was forced to find her best mental resources to beat Julia Goerges 4-6 6-4 6-4. The 2008 Australian Open champion might want to step up a gear in the next round when she takes on Andrea Petkovic for a place in the quarter-finals.
Belarus's Viktoria Azarenka will also have tough task when she plays in-form China's Na Li, who just beat Clijsters to clinch the Sydney title.
How about Tommy Robredo, or Nicolas Almagro, who both have to meet successively Roger Federer and Novak Djokovic on Day 7 at the Australian Open.
Petkovic might be on the verge of making her breakthrough, which would erase the bad memories of 2008 Australian Open.
In her first round match, the No. 30 seed suffered a cruciate ligament rupture in her right knee after only two minutes of play, then ranked No. 92, she was out of the sport for eight months. In any case, here she is bidding to become the first German to reach the quarters at the Australian Open since Steffi Graf in 1999.
Meanwhile, Sharapova is shooting for her first major quarterfinal since 2009 French Open.
The former World No.1 situation contrasts strongly with that of the World No. 33. Petkovic barely had to play any tennis in her third round match against Venus Williams. The American retired with an injury that had been sustained in her match against Sandra Zahlavova.
At the same, the time three-time Grand Slam Champion, is going to be the more fatigued of the two competitors after having been forced to play three sets against Julia Georges.
The pairing have only met once previously in official competition. The match, which Sharapova won convincingly (6-3 6-1), took place on a hard court in Cincinnati last season.
One last interesting stat surrounding the No. 14 seed.
On Friday, the Russian improved her career three-set record to 100-36 by defeating Julia Goerges in the third round. She is the only player in the Top 100 with a success rate of more than 70 percent in three-set matches (73.5 percent).
The message for Petkovic, do it in two.
When will a Chinese win a major? Maybe sooner then many think.
Li, who already reached the semifinals In Melbourne last year, has emerged as a paragon of steadiness on this Australian summer while being no less explosive.
The 9th seed is now on an eight match streak, taking in her feat of beating Kim Clijsters to become the first Chinese player to win a WTA Premier title in Sydney last week and three routine wins in Melbourne.
In fact, Li has dropped just 14 games so far, the fewest of any player in this half of the draw.
Meanwhile, Azarenka had the bad luck of falling to Serena Williams here over the last three years, with last year being the closest she has been in beating the American leading 6-4 4-0.
Belarus has without a doubt all the tools to win a major. It might very well happen this year.
Li leads Azarenka 2-1 in their head-to-head meetings, with the Belarus winning their last encounter last year in Montreal 6-3 6-3.
Of late, Li has proven to be serene under pressure, so if things get tight, she could have the edge.
The one who will go for her shots will prevail, but all ingredients suggests it should be a mouthwatering encounter.
The match of the day on the men's side will feature Roddick against Wawrinka.
The Swiss is doing everything in his power to follow his master countryman Federer.
As a matter of fact he decided to hire the 29-year old's former coach Peter Lundgren, who helped him climb to the World No. 9 spot, his highest rank ever.
The 19th seed already has one of the best one handed backhand in the world , while Lundgren helped Wawrinka improve his serve to become the player he is now.
That is why Andy Roddick faces a difficult clash against the World No. 19 on Sunday.
The American bounced back from a first set loss to defeat Robin Haase in the third round, and seems to be in decent form, while the St. Barthelemy native has won all of his matches in Melbourne in straight sets. His most recent outing, where he defeated Gael Monfils, indicates that he could really threaten the Austin resident on Sunday.
This pairing has a slightly unusual head to head history. Wawrinka leads 2-1, though both of his victories came through first set retirements.
The only competitive match to be completed between the pair was played on a hard court in 2009, with Roddick securing a comfortable 6-4 6-4 6-2 win when the USA received Switzerland in a Davis Cup first round encounter back in 2009.
This match could easily turn out to be a grueling five-setter.
One of the Spanish men in action on Sunday is Tommy Robredo. The former French Open quarterfinalist is the only unseeded player through to the Round of 16 in the bottom half of the draw. He faces Roger Federer on Rod Laver Arena. Can he stop the Swiss from progressing one step closer to defending his title?
Very bad news in the fact that the Swiss has won his last nine meetings with Robredo. In that time, the Spanish player has won just two sets versus his Swiss rival.
Elsewehere, the defending champion advanced to the Round of 16 here for the 10th straight year, when playing his 12th straight Australian Open.
Moreover, the last time Federer failed to reach the quarterfinals at a Grand Slam was at 2004 Roland Garros.
One more note about the second seed is he is bidding to contest his 27th successive Grand Slam quarterfinal and equal Jimmy Connors’ record for the most consecutive Grand Slam quarterfinal appearances in the Open Era.
As for the former World No. 5, he will be bidding to reach the quarterfinals here and equal his best result at a major. One very impressive record from the former top 10 member, is he has contested the last 41 Grand Slam events (including this one).
Djokovic is the third favorite to win this year's first major—behind Nadal and Federer, as he will be trying to win his second Australian Open.
The man from Serbia advanced to the Round of 16 for the fifth straight year when he benefited from the retirement of countryman Viktor Troicki.
The World No. 3, who helped his country win the Davis Cup for the first time last December, also reached the quarterfinals or better at 13 of the last 15 Grand Slam events he has contested which is a very impressive record.
On the other side of the net Almagro will try to to reach the last eight stage here for the first time, while his best Grand Slam result is a quarterfinal finish at Roland Garros in 2008 .
In their only previous meeting, the Murcia resident was forced to retire with a right wrist injury.
Almagro has the stamina and determination to cause some damage with his amazing backhand, however Djokovic, who has more weapons than the Spaniard, will be favorite to win this match.