Second round won't be easy for Serena, Federer, Bouchard

— -- The theme on a loaded Wednesday at Melbourne Park is variety, as a striking array of matches will be featured on the show courts. Here are three choice matchups:

Top-seeded Serena Williams versus No. 90 Su-Wei Hsieh

Williams got out of a jam in her opening match, battling through to a victory against the highest-ranked unseeded player, hard-hitting Italian Camila Giorgi. On Wednesday, Serena will have to recalibrate her game as she faces a fellow Grand Slam champion.

All right, so Hsieh earned her two Grand Slam titles in doubles (Williams, incidentally, has 13 of those to go with her 21 in singles), but Williams still needs to be alert against the 30-year-old Hsieh, who hits the ball with two-handed shots off both wings. It's a slight advantage for Hsieh, because she has never played against Williams, and two-handed groundstrokes are notoriously hard to read.

The book on Williams is that an opponent is better off trying to go around rather than through her, better off forcing her to generate her own pace rather than offering her velocity to work with. Hsieh has the kind of game that can implement that plan; whether it pays off or not is a different matter. There's a reason the book on Williams is a slim volume. Hsieh is 0-10 against top-10 opponents, and has just one win versus a top-20 player.

No. 2 seed Roger Federer versus No. 35 Alexandr Dolgopolov

The problem with being such a big star that your peers will hop on a plane to go practice at your place is that it can backfire if you have to play them shortly thereafter. That's where Federer finds himself with Dolgopolov, who worked out with the all-time Grand Slam singles champ at Federer's base in Dubai just weeks ago.

"I think it's going to be very tough, to be honest," Federer told reporters of the matchup after his first-round win. "We had some great practice sessions together there [in Dubai] ... He's got the fitness, the power, the speed, tennis IQ, all that. It's going to be a big challenge."

Dolgopolov is a mercurial but inconsistent player. He's been ranked as high as No. 13, but is now 35th. At 27, though, he's still young, quick and explosive. One of his assets will be his penchant for playing quickly and forcing points to swift conclusion. Federer himself has evolved into a much more aggressive, offense-minded player these days, so this could become an entertaining, old-fashioned shootout. The two have played just once in the past five years; Federer won that 2014 match handily.

No. 4 seed Agnieszka Radwanska versus No. 37 Eugenie Bouchard

This is an intriguing matchup of two former Wimbledon finalists in resurgence, although the orders of magnitude differ.

Radwanska, who struggled through the early part of 2015, has won four WTA titles since the US Open, including the WTA Tour Finals. Bouchard, meanwhile, is content just to be winning matches. After she appeared to be rounding back into form from a horrendous 2015 slump at the US Open, she fell in a locker room and suffered a concussion that kept her out of action for the remainder of the year. (Bouchard has a lawsuit pending against the USTA).

Psychology aside, this is a compelling matchup of styles. At her best in her breakout 2014 season, Bouchard took the ball on the rise and belted relatively flat shots at sharp angles, taking time away from her opponents. She set herself up for easy placements and putaways. Radwanska is a quintessential counter-puncher, adept at turning the tables on opponents who try to blast through her. She is a great retriever and has a great gift for changing pace.

In their only previous meeting in 2014, Radwanska picked Bouchard apart in straight sets on the red clay of Madrid.