Welcome to the turn at the US Open! As we enter the second week of the year’s final major, we have eight awesome round of 16 contests featuring four former Open winners, a 2012 Olympic gold medalist in Andy Murray and another former Grand Slam tournament finalist in Tomas Berdych (2010 Wimbledon). Additionally, in the third round we bid farewell to 2001 US Open champion Lleyton Hewitt, as the men’s elite are beginning to fall fast heading to – weather permitting – a singles final next Sunday inside Arthur Ashe Stadium.

After this round, we will also be saying goodbye to this current format of Top Five Factors. As the match numbers dwindle, our feature series shifts to deeper looks into individual matches and the historic impact that rides upon their outcomes.

We may even open things up to our Facebook commenters to suggest what the Top Five Factors for one of our singles finals will be. After all, the fans and their reaction form the memories of each US Open just as much as the players or members of the media. In the spirit of our first-ever Daily Challenge trivia contest on USOpen.org in 2012, this year’s Open is for the thinking fan.

Here are the five must-see men’s matches for the fourth round:

Roger Federer (1) vs. Mardy Fish (23)

The 17-time Grand Slam champion began his ascension to the apex of the men’s tennis landscape with a victory at Wimbledon in 2003, his first major title. Along the way to defeating Mark Philippoussis in the final, Federer found himself in a match for the first time in his professional career against a 21-year-old named Mardy Fish. In the match, the American gave the fourth-seeded Federer a scare, firing two-handed backhand winners and taking the third set. Federer rebounded to ultimately win 6-3, 6-1, 4-6, 6-1 and developed into a technical maestro, more accomplished and decorated than any male tennis player of any age.

Fish is still looking for that first major victory, and he’ll have to go through Federer for the first time at a Grand Slam event since that fateful Wimbledon nine years ago to reach it. Lifetime, Fish is just 1-8 against Federer, including a 6-3, 7-6 loss a few weeks ago in the quarterfinals of Cincinnati (where Federer used a victory over Fish once more to take home a title). He’s played a lot of tennis in the first week, needing 12 sets -- four of them ending in tiebreakers -- to advance this far. Federer, as is his custom anytime he comes to Queens, has faced the minimum in one-sided wins over Donald Young, Bjorn Phau and No. 25 Fernando Verdasco. A plus in Fish’s favor is that the heat that scorched the courts over the weekend will have subsided by Monday afternoon, forecasts calling for cool, cloudy cover. There are no secrets between the two in terms of preparation: Mardy Fish will have to play the match of his life to beat Federer, five times an Open champion with a sixth in sight.

Juan Martin del Potro (7) vs. Andy Roddick (20)

Keep the star-spangled sneakers out of storage, and bring back Brooklyn, Blanche and Jerry. In this match, Roddick looks to delay retirement one more time against another banged-up one-time Open winner in del Potro. With Roddick, it’s the shoulder – “It's not great. But, you know, it's good enough. I got max a week of tennis left, so it's good enough for that.” For del Potro, it’s the wrist that has arrested his career since his breakthrough in 2009 plus a tweaked knee from his Sunday win over fellow Argentine Leonardo Mayer (despite del Potro’s own assertions that he is in “perfect” shape, eyeing his play leads to the belief that he’s choosing the option of nondisclosure when speaking to the media).

Roddick had never beaten del Potro until a 2011 win in Memphis, 6-3, 6-4, their last match. Their matches as a rule have been slam-bang affairs, Roddick the slightly better server and del Potro the superior returner. The immediate thought will be that Roddick can channel the cheers and rise to a higher level, all the while opponents are left jittered by the noise.

It affected Bernard Tomic, young and inconsistent, yet a much better player than he showed inside Arthur Ashe Stadium. It did not effect Fabio Fognini, whose relentless passing shots and athleticism came close to ending it all on Sunday. Nothing should spook a player that took on Federer and ended his 40-match New York win streak on the biggest platform of all back in 2009. That he’ll all-but-certainly get a few additional hours to rest with this match possibly scheduled for Tuesday night and with a fuller house and out of the heat, all advantages to Roddick.

Novak Djokovic (2) vs. Stanislas Wawrinka (18)

After a methodical triumph over no. 31 Julien Benneateau 6-3, 6-2, 6-2 on Sunday, the defending Open champ Djokovic must defeat one Swiss, Wawrinka, before a potential shot at another in Federer in the final. Fortunate news for Djokovic is that he draws an opponent that he not only knows well – 14 head-to-head matchups in eight years touring – but also beats nearly every time out, to a 12-2 career mark. The story behind the only time they’ve met at a major lends itself to some historic significance as it was the qualifying tournament for the 2005 Australian Open, when a 17-year-old Djokovic beat Wawrinka 6-3, 6-1 to reach his first main draw at a Grand Slam. The two have almost exclusively played on clay against one another since Djokovic’s rise to No. 1, including a 7-6 (5), 6-4 win for the Serb this year in Madrid.

Wawrinka has appeared in four ATP semifinals (Cincinnati, Estoril, Acapulco and Buenos Aires) and two additional quarterfinals (Monte Carlo, Chennai) and enjoyed the honor of being Switzerland’s national flag bearer at the 2012 London Games. Beating Djokovic and getting himself back into a Grand Slam quarterfinal would trump all of that, and the key to doing so will be establishing his rocket of a backhand. This signature stroke has neutralized opponents thus far in Wawrinka’s Open trek, as he can extend his reach while still pulling off powerful, penetrating shots. Wawrinka also merits upset attention because he’s done it before, beating Murray in this same round two years ago.

David Ferrer (4) vs. Richard Gasquet (13)

For the second time in 2012, these two Europeans find themselves in a round of 16 match in a Grand Slam event. Back in January, it was Gasquet coughing up one break point after the next and Ferrer giving no openings for Gasquet’s deftly spotted backhand shots. Gasquet has provided the knockout in the last two rounds to a pair of prominent American college players – Bradley Klahn and Steve Johnson – putting himself a win away from his first US Open quarterfinal berth.

Ferrer has been as far as the semifinals in 2007 and is running out of opportunities to return to that threshold. His has arguably been the more arduous travel to the round of 16, having to beat a very dangerous Kevin Anderson in the first match and then Hewitt on Sunday. Ferrer is 7-1 all-time against the Frenchman, and the defensive Spaniard will return everything Gasquet tries to position over the net, and after match point, he’ll be returning to the quarterfinals.

Marin Cilic (12) vs. Martin Klizan

Klizan has been beyond expectation, acing tests from two of France’s finest players – No. 32 Jeremy Chardy and No. 5 Jo-Wilfried Tsonga – and a very deft opening round opponent in Colombia’s Alejandro Falla, who nearly beat Federer at Wimbledon two years ago in that first round. Klizan’s winning has had a formula: attack the net, getting serves in and baiting opponents into making errors while severely limiting his own.

The Croatian Cilic is still just 23 years of age and playing in his fifth US Open main draw (not counting 2006 and 2007, when he lost in the qualifying tournament), and has been seeded each year since 2008. He faced elimination at the jump against Marinko Matosevic, dropping the first two sets before saving it 5-7, 2-6, 6-4, 6-2, 6-4. The loss would have been Cilic’s first ever one-and-done in Flushing Meadows, and the follow-up was more of the same, as Cilic needed five sets after blowing a lead to another unseeded player, Daniel Brands of Germany, 6-3, 6-2, 5-7, 4-6, 7-5. Cilic also made headlines in 2012 by having the second-longest match in the history of Wimbledon, defeating Sam Querrey in five sets, 7–6(6), 6–4, 6–7(2), 6–7(3), 17–15. Call him the “marathon man.”

***

Follow USOpen.org's Nicholas J. Walz on Twitter: @ifwalzcouldtalk

• Roger Federer Bio
• Andy Roddick Bio
• Mardy Fish Bio
• Juan Martin Del Potro Bio