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'Battle' the key word in these fourth-round matches

The Australian Open quarterfinals will be set by the end of Monday. It's unlikely that anyone will put on the kind of clinic Roger Federer or Serena Williams staged Sunday or, for that matter, win a match despite churning out 100 unforced errors the way Novak Djokovic did. This ought to be a day consisting mostly of nip-and-tuck battles between well-matched opponents.

No. 4 seed Stan Wawrinka versus No. 13 Milos Raonic

By the end of the day, one of these two heavy hitters is no longer going to be undefeated in 2016. Odds are that Wawrinka will prevail, as he won the event in 2014 and lost to eventual champ Djokovic in last year's semifinals.

It's not that Raonic has a particular weakness that Wawrinka can exploit. It's just that Wawrinka seems to be in Raonic's head. In big matches at major events, that counts for a lot.

Wawrinka is 4-0 against Raonic. The only set he lost to the 25-year-old Canadian is the first one they played, in Cincinnati in 2012. Since then, Wawrinka has reeled off eight straight sets, a whopping five of them decided by tiebreakers. No wonder he has told the media, "In general, I always find some solution to break his serve, even [though it] is really tough."

But is this a new Raonic? He's certainly off to a great start. On the same day that Wawrinka beat No. 44 Borna Coric for the title in Chennai a few weeks ago, Raonic was raining down aces in an upset of Federer in the Brisbane final.

Raonic's new coach, Carlos Moya, is certainly going to remind his protégé that anyone who can beat Federer can probably beat the other Swiss guy too.

No. 15 Madison Keys versus No. 133 Zhang Shuai

Zhang held an ignominious record before this tournament -- she has played in 14 majors without winning a match. But now the 27-year-old underdog is the first qualifier to get this far in the tournament since Yanina Wickmayer in 2010, and she hasn't done that with smoke and mirrors.

Zhang's run began with an upset of No. 2 seed Simona Halep; then, in her next two matches, she took down two top-50 players and lost only 10 games and no sets. By contrast, Keys has struggled through perilous three-set matches in her past two rounds. Her consolation: Her opponents, Yaroslava Shvedova and No. 20 seed Ana Ivanovic, are also quality players.

Keys certainly is battle-hardened, and that will be an asset should the match go to three long sets. Keys is a mercurial, explosive player who can play lights-out tennis for lengthy periods, but also has a tendency to lose the plot and litter the court with errors. Zhang likely will play a patient, prudent game.

These women have split the four matches they've played, but that's deceptive because of the seven-year age difference. It's been almost two years since they last met and Keys was 18 at the time.

No. 8 David Ferrer versus No. 10 John Isner

No one likes to look across the net and see Ferrer. Traditionally, it is guaranteed to be a long and likely futile day. That's not as often a case now; Ferrer is 33 with a lot of miles on the odometer. This isn't the toughest imaginable obstacle standing in the way of Isner making a Grand Slam quarterfinal for just the second time in his career. He knows this is a big opportunity.

The 6-foot-10 American, who is a full foot taller than Ferrer, goes the way of his serve. That that delivery has been lethal. Isner has 101 aces so far in Melbourne and, more importantly, hasn't allowed a break point. Returning serve is one of Ferrer's strong points, but the height disadvantage is glaring. Isner will try to exploit Ferrer's lack of reach with his serve selection and placement.

Ferrer is in good shape going into this match. He has ripped through his first three opponents without losing a set and holds a 6-1 advantage over Isner. Five of those matches (including Isner's lone win) were contested on Isner's preferred hard courts.

Isner has his work cut out for him, but one of the immutable laws of tennis is that if your serve is big enough, anything is possible. And Isner's serve is as big as Ferrer's heart. We'll see which is bigger Monday.