Rafael Nadal and Fernando Gonzalez will square off in the fourth round of the Australian Open 2009 on Monday afternoon, but the action on Rod Laver Arena will be kicked off Gilles Simon and Gael Monfils.

It will be the first career head-to-head meeting on between the two Frenchman, and count on this showdown being by far the most difficult test for each participant so far in this Australian Open. Simon has dropped only one set en route to the fourth round, and even then he was never seriously troubled after losing the first set to last remaining Aussie Chris Guccione in round two. The No. 6 seed sandwiched that victory with straight-set wins over Pablo Andjuar in his opener and over Mario Ancic on Saturday. It looks like Simon not missed a step after his stunning emergence into ATP prominence last season. He was not even seeded in Melbourne a year ago, and now he is the No. 8 player in the world. Highlights from the Simon’s breakout 2008 campaign included titles in Casablanca, Indianapolis, and Bucharest, a runner-up finish at the Masters Series Madrid, and semifinal performances at the Masters Series Canada and year-end Masters Cup.

An easy road so far Down Under? A career-year in 2008? Sounds a lot like Monfils, as well. The No. 13 player in the world dismantled Martin Vassallo Arguello in the first round before knocking out Stefan Koubek in four sets, again with little difficulty. Monfils is coming off an extremely impressive performance in round three, in which he sent No. 17 seed Nicolas Almagro packing in straight sets. This Frenchman might not have had quite as much success as Simon last year, but he’s arguably on an even bigger hot streak at the moment. In the second half of 2008, following a semifinal showing at the French Open, Monfils reached the quarterfinals of the Olympics and the fourth round of the U.S. Open prior to a stellar indoor swing. Already this year he has an upset of Rafael Nadal (in Doha, Qatar) under his belt.

Whose brilliant form will carry into the Australian Open quarterfinals? Monfils definitely has the edge in terms of Grand Slam experience. He won multiple major titles as a junior and should have his recent Roland Garros performance still fresh in his mind. Simon, on the other hand, had never made it pas the third round of slam prior to his win over Ancic. The advantage in an extremely tight marathon match, however, should go to Simon, who has made a living off tense encounters. Just one example is his run to the Madrid final, which featured five straight three-set matches, four (including one against Nadal) of which went to third-set tiebreakers.

Little separates these two men on paper, and even less separates them on the court. Simon and Monfils are two of the best fighters and most impressive counter-punchers on the tour. This clash is going to be all about keeping balls in play and turning defense into offense. Simon’s down-the-line backhand is becoming a real weapon, but Monfils does goes from the defender to the aggressor better than almost everyone other than Nadal.

While Simon vs. Monfils could be the best match of the day, Aussie fans will surely be anticipating Nadal vs. Gonzalez even more. That’s because both men complete dominated third-round headlines.

Nadal opened his fortnight in Melbourne with a 6-0, 6-2, 6-2 destruction of Christophe Rochus. He then dismissed Roko Karanusic 6-2, 6-3, 6-2 before really raising his level to other-worldly standards. On Saturday night, Nadal fired a ridiculous 53 winners to eight unforced errors as he dismantled Tommy Haas 6-4, 6-2, 6-2.

So much for any doubts about Nadal’s early season form? Yeah, I think so.

Those questions had surfaced after the world No. 1 returned from knee tendinitis and lost to Andy Murray in an exhibition and to Gael Monfils in Doha. Now, however, it looks like Nadal could be nearing the level of play that carried him to titles at the French Open, Wimbledon, and the Beijing Olympics in 2008.

Unlike Nadal’s pleasant stroll into the fourth round, Gonzalez has already survived two five-setters Down Under. Much to the chagrin of the Australian faithful in Rod Laver Arena, the Chilean outlasted Lleyton Hewitt 5-7, 6-2, 6-2, 3-6, 6-3 in a first-round showdown. Gonzalez went on to dispartch Guillermo Canas, setting up the epic encounter with Richard Gasquet. Gonzalez pulled out that instant classic 3-6, 3-6, 7-6(10), 6-2, 12-10 and even saved a match point in the third set.

It’s been an impressive display so far for the No. 13 seed, who is still a solid 14th in the world at 28 years old. Gonzalez appears to be picking up right where he left off following a stellar 2008 campaign, which was highlighted by titles in Vina Del Mar and Munich, a quarterfinal appearance at the French Open, and a silver medal at the Olympics (lost the gold medal match to Nadal).

All of that would suggest it might be wise not to discount Gonzalez’s chances against Nadal. So would the underdog’s almost .500 career record in head-to-head meetings with his opponent. Nadal has won just four of seven previous encounters. But when you take into account Nadal’s flawless performance against Haas and Gonzalez’s grueling, one-for-the-ages clash against Gasquet, it’s hard to justify giving Gonzo any chance.