Quarterfinals preview: Now, it gets serious at the Australian Open

— -- Anyone who has already penciled in that Novak Djokovic- Roger Federer semifinal clash at the Australian Open ought to pause a moment: In Tuesday's quarterfinals, both will face opponents who beat them the last time they met in Grand Slam events.

Keeping that in mind, let's take a look at all four top-half quarterfinals:

No. 1 seed Serena Williams versus No. 5 Maria Sharapova

ESPN2/WatchESPN: Watch Serena-Sharapova match live, 9 p.m. ET

The buzz in Melbourne now is all about Sharapova's recent serving efficiency and how it might finally enable her finally to break her nightmarish 17-match losing streak against Williams. Perhaps that will happen, because for a while now, the odds have been that some day, somehow, somewhere, Maria Sharapova will once again beat Serena Williams.

There are a number of reasons to believe that such a thing will happen. At 34, Williams is about five-and-a-half years older than Sharapova at a time when the discrepancy is really starting to work against Williams. It's also possible that as much as Williams loves to beat Sharapova (it goes back to that surprise upset Sharapova pulled on Williams in the 2004 Wimbledon final), she will go out there one day in an off-kilter, "been there, done that" mood and play poorly.

Or maybe one day Sharapova will simply play a perfect match, which is what it will take for her to beat Williams.

Could it be Tuesday? It's unlikely, for while Sharapova leads the tournament in aces (52, more than twice Williams's 25), she ranks behind Williams in first-serve points won (86 to 82 percent).

No. 1 Novak Djokovic versus No. 7 Kei Nishikori

ESPN2/WatchESPN: Djokovic-Nishikori match live, 3 a.m. ET

No one has had good reason to feel optimistic before meeting Djokovic these days, but Nishikori is entitled to feel a twinge of hope.

Djokovic committed 100 unforced errors in a narrow escape in his last match, and Nishikori won the last time they met at a major (the 2014 US Open semifinals). If Djokovic has suddenly misplaced his game, or his confidence, Nishikori surely will have a chance.

Djokovic has won four straight against Nishikori since that Open semifinal loss, but two of those were three-setters that went the distance. Nishikori plays a relatively flat game and likes to take the ball on the rise from inside the court. Those are good tactics against Djokovic, who loves to yank people all over the back of the court.

Nishikori has a reputation as a gritty player, yet his performance at the majors has been tepid. Although he has been ranked as high as No. 4, he's made just that lone semifinal.

No. 3 Roger Federer versus No. 6 Tomas Berdych

ESPN2/WatchESPN: Watch Federer-Berdych match live (not before 10:30 p.m. ET)

A quick glance at the head-to-head here would be highly deceptive. Sure, Federer leads by a commanding 15-6 and he's won the last four meetings (not even dropping a set or needing a tiebreaker in the last three of those matches). But there's this:

Federer is 34, and just over 6 feet tall. Berdych is 30 and 6-foot-4, blessed with superior power, and has beaten Federer the last two times they met at Grand Slam events (Wimbledon in 2010 and the US Open in 2012).

Berdych has good reason to feel he can go right at Federer and overpower him, and by now he's had ample time adjust to Federer's new, more aggressive style. That will give Federer and his new coach, Ivan Ljubicic, plenty to think about.

Serving success will be important in this match. Federer has been the most successful ATP player in the tournament when it comes to winning first-serve points (84 percent), but Berdych isn't far off at 81 percent. Neither man cracked the tournament top 20 in second-serve points won, though, so whichever one is more successful in that department may have the winning edge.

No. 4 Agnieszka Radwanska versus No. 10 Carla Suarez Navarro

ESPN3/WatchESPN: Watch Radwanska-Suarez Navarro match live, 7 p.m. ET

This match will be a stunning contrast to the other women's quarterfinal, and a terrific advertisement for the current diversity in the WTA game. Radwanska and Suarez Navarro are two of the most interesting, creative players of this or any other era, and how they've made their way in a game presently dominated by power baseliners is a tribute to their talent.

Radwanska has been on fire since the US Open. She's won 26 of her last 30 matches, including the year-end WTA championships. She was down 2-5 in the third set in her fourth-round match, but recovered to win. Players who make such comebacks are always especially dangerous, as if Suarez Navarro didn't already have her hands full with Radwanska's clever, off-pace game.

Radwanska herself is accustomed to dealing with players who come straight at her with pace and power. But Suarez has touch. She hits her backhand with one hand and likes to slice and work her way to the net. Radwanska, a counter-puncher who likes to mix up the pace, will feel just as uncomfortable as her opponent.

Radwanska leads 2-1, but they split their last two meetings. Both went three sets and included a love set by the winner.