Americans Madison Keys, John Isner face fourth-round tests

Three of the top American players hoping to make a big impact Down Under will be helping complete the third round Saturday at Melbourne Park. Here's a closer look at what lies in store for them:

No. 10 seed John Isner versus No. 18 Feliciano Lopez

This pairing might not feature a Big Four player, but it promises to be a splendid clash of attacking styles between explosive players treading water in roughly the same section of the talent pool.

Fans in the United States have been waiting for Isner to ride that monster serve of his deep into a major since forever; Lopez partisans remain convinced that the late-blooming 34-year-old veteran still can slice his way through any Slam draw to make a grand statement.

The fact that 30-year-old Isner and Lopez are like-minded players operating at roughly the same level has infused their meetings with urgency, and the way their strengths and weaknesses dovetail helps account for the closeness of their rivalry.

They've split six matches, each of them beating the other on his preferred surface. Ten of the 19 sets they've played were decided by tiebreakers (6-4 to Isner), including six of the past nine. Those six were all played on grass, a surface that can render each of their serves virtually unbreakable, especially in light of their so-so prowess as returners.

Look for the player who returns better, or gets more looks at second serves, to advance.

No. 15 Madison Keys versus No. 20 Ana Ivanovic

Back-to-back matches against a woman who recorded the only "golden set" of the Open era and one who has won a major and has held the top ranking isn't exactly what a player hopes for in second- and third-round matches, but that's the assignment the 20-year-old American was given. Keys completed half of it successfully with a gritty three-set win against Yaroslava Shvedova on Thursday; now comes the harder part.

Although Ivanovic has never quite returned to the heights she reached when she won the French Open and hit No. 1 in 2008, she has gone through periods when she's been outstanding and others when she's merely been very good.

Ivanovic appears to be in one of her lesser phases now, but the problem for Keys is you never know where and how Ivanovic might snap out of it. Serena Williams certainly found out when Ivanovic's game suddenly caught fire at the start of 2014 when Ivanovic knocked Williams out of this tournament in the fourth round.

Keys and Ivanovic have met only once -- on the red clay, where Ivanovic is at her best, in Madrid in 2014. Keys was still a teenager then, but she is stronger and more experienced now. To win, Keys will have to use her abundant power to contain and overwhelm a precise and well-balanced opponent who excels at the rally game and counter punching.

No. 8 David Ferrer versus No. 31 Steve Johnson

Ferrer has been to the fourth round for five consecutive years; Johnson has not been that far at any major. Ferrer has won 26 ATP Tour titles; Johnson has yet to claim his first.

The odds clearly favor Ferrer, but for this: At age 33, the fangs and claws of the man dubbed "The Little Beast" have been ground down by constant use, and at times his famous stamina seems diminished. By contrast, Johnson is just 26 and on the rise. A diligent, patient student of the game who didn't join the pro tour until 2012, Johnson won more tour matches (36) last year than in his three previous years combined.

Despite the strides he has taken, Johnson still must show that his old-school game, which is long on smart, situational play and the use of his slice, can hold up against elite players. In that regard, his first and only meeting with Ferrer was encouraging. Johnson reached his first ATP final last fall in Vienna, where he played an excellent match before faltering as Ferrer won it, 4-6, 6-4, 7-5.