Defending champion Kim Clijsters beat Caroline Wozniacki 6-3, 7-6 (4), ensuring Wozniacki will lose the No. 1 ranking. In a replay of the 2009 US Open final won by Clijsters in straight sets, the Belgian player had an early service break in the second set before breaking Wozniacki's serve again to lead 4-2.
(11) Del Potro vs. (3) Federer
In this highly anticipated rematch of the 2009 US Open final, Roger Federer will surely try to remind Argentina's Juan Martin del Potro just why he's won more Grand Slams than any other player. But del Potro, who has only dropped one set in Melbourne, has flown a bit under the radar and could shock Federer once again.
(2) Nadal vs. (7) Berdych
Spain's Rafael Nadal hasn't dropped a set yet, but his luck may run out against the always-challenging Tomas Berdych of the Czech Republic. However, this matchup has a twist in that Berdych lost the support of the crowd after what was perceived as unsportsmanlike behavior against Spaniard Nicolas Almagro in their fourth-round match. Look for Nadal to use this to his advantage.
(2) Kvitova vs. Errani
2011 Wimbledon champ Petra Kvitova is eyeing another Slam title, as well as the No. 1 ranking. With Serena Williams gone, on paper the No. 2 seed has a straight shot to the final. But the gutty unseeded Italian Sara Errani has made her way into the quarterfinals. Will Kvitova continue her path to becoming No. 1 or will Errani, at 24, score a huge breakthrough win?
(3) Sharapova vs. Makarova
Maria Sharapova won the Australian Open in 2008 and would like nothing more than to win another Slam title. Standing in her way is fellow Russian Ekaterina Makarova. Sharapova leads their series 2-0, winning both matches on clay in 2011, but can Makarova carry the momentum of knocking off five-time Australian Open champion Serena Williams with her?
(24) Nishikori vs. (4) Murray
Many have been waiting for Japan's Kei Nishikori to make his big break. Late last year he notched a victory over a weary Novak Djokovic in Basel before losing in the final to Roger Federer. And, in the fourth round in Melbourne, 22-year-old Nishikori took down Jo-Wilfried Tsonga in five sets to meet two-time Australian Open finalist Andy Murray. Murray beat Nishikori in a decisive two sets in their only meeting last fall in Shanghai.
(1) Novak Djokovic vs. (5) David Ferrer
Top-seed Novak Djokovic will face David Ferrer coming off his first true challenge of the tournament -- a four-set win over Australian Lleyton Hewitt. Ferrer, much like Hewitt, will if nothing else make Djokovic work much harder than he did the first three rounds where he barely broke a sweat. Djokovic leads their head-to-head series 6-5, so Ferrer clearly knows how to beat the Serb.
(3) Azarenka def. (8) Radwanska
Victoria Azarenka reached the semifinals when she held her nerve despite trouble with her serve in a 6-7 (0), 6-0, 6-2 win over Agnieszka Radwanska. The victory gave Azarenka, 22, her 10th straight win this season.