It's not just Federer versus Nadal: Previewing the men's semi slugfests

PARIS -- And then, there were four. After a rain delay pushed the remaining men's quarterfinals to Thursday, Dominic Thiem and Novak Djokovic advanced to face one another in Friday's semifinals. The other clash is between Roger Federer and Rafael Nadal (12:50 p.m. local time start, followed by Djoker-Thiem). Our experts break down what to expect in the 2019 French Open men's semifinals.

Which underdog is more likely to win his semifinal match, Dominic Thiem or Roger Federer?

ESPN.com tennis writer Peter Bodo: The surprise guest at this Big 3 party is Thiem, and like many players, he's been overshadowed during clay season by the relentless focus on Federer's return to red dirt. But Thiem has a better chance of winning his semifinal than does Federer. The 25-year-old Austrian wobbled a little in the final run-up to Roland Garros, but caught fire and crushed his past two opponents there (No. 14 seed Gael Monfils and No. 10 Karen Khachanov).

He has been a French Open semifinalist or better for four straight years, something none of his rivals can claim. He's ready for a breakthrough; plus, he's already beaten Djokovic here (2017). He's extremely diligent about his fitness and strong as an ox, so he's capable of hitting through Djokovic's vaunted defenses, even in a super-long match.

ESPN.com contributor Simon Cambers: Thiem. Mainly because he's beaten Djokovic on clay, at Roland Garros before, and his game matches up better against Novak than Roger's does against Rafa, on this surface. He now believes he can win this title.

ESPN senior writer Bonnie D. Ford: I have to go with Thiem. The stats against both are daunting, and Nadal and Djokovic currently appear invulnerable, but if someone's going to benefit from the law of averages, I'll give the edge to the steady Austrian who's been on the brink for a while now.

How could the unequal amount of rest affect both matches?

Bodo: Rest, or lack thereof, is unlikely to be a factor, as neither Djokovic nor Thiem appears to have a physical weakness or injury that might have benefited from the traditional day of rest. The important thing is these two will play each other and operate with the same disadvantage.

Neither Federer nor Nadal has had extremely taxing matches, so the extra day of rest may, if anything, disrupt their rhythm. Extra rest can be a momentum-stopper for someone playing as well as Nadal and Federer. Professional as they are, worries do creep into idle minds, and even tennis titans can burn up a lot of energy waiting and wondering. But they are playing each other, so again: level playing field.

Cambers: It's not going to help Djokovic and Thiem, but at least both have had pretty much the same rest. Providing the weather holds, they know they'll have a day off Saturday, so they'll go full throttle against each other. Federer and Nadal could be thrown by the two days off, but you've got to think Federer, in particular, will have needed that.

Ford: Extra "rest" can be a funny thing sometimes, interrupting the routine and rhythm these thoroughbreds crave. Federer and Nadal, being older players and consummate pros, will know how to profit from it. Both Djokovic and Thiem had efficient quarterfinal matches of 2 hours, 9 minutes, and 1 hour, 47 minutes, respectively -- so I don't expect either match to be markedly affected by fatigue. More important will be the players' ability to deal with the blustery weather in the forecast.

In Nadal-Federer matchup, what is most at stake for both players?

Bodo: Nadal already has 11 French Open titles, 17 Grand Slam singles crowns and the undisputed title of "King of Clay." Federer has 20 majors and is already perceived as the GOAT in the eyes of many; should he win this match and go on to claim the title, that would probably become indisputable. The achievement, at 37, would go down as perhaps his greatest, and certainly most unexpected, accomplishment.

A win by Nadal keeps alive the hopes he has of catching Federer and out-distancing Djokovic in the Grand Slam title derby. Nadal already leads his series with Federer 23-15. Another win over his career rival in a major only increases the validity of the question first posed years ago by former French Open champ Mats Wilander: "How can Federer be the GOAT if there's a guy he can't even beat in his own era?"

Cambers: All the pressure is on Nadal, who needs to win to maintain his aura on clay, having lost his past five matches to Federer, albeit on other surfaces. There is no external pressure at all on Federer, who can just swing freely, which makes him dangerous.

Ford: Percentage-wise for his legacy, Federer probably has more to gain by coming back to clay courts after three seasons away and potentially running the table at Roland Garros. Another title match for Nadal -- while extraordinary -- wouldn't materially change the way history regards him.

Novak Djokovic has cruised through this tournament so far. What will it take for Thiem to break through?

Bodo: Dueling a guy who basically does the same things as you but better is usually a sure ticket to Loserville. Thiem and Djokovic play different versions of the same, baseline-hugging game. The main difference is that where Djokovic is quick, versatile and adaptable, Thiem is a bit slower, less capable of transitioning from defense to offense and committed to hitting straight through opponents.

Thiem will have to get his first serve into play -- he's No. 2 on the Infosys Slam Leaderboard in winning his first-serve points, but Djokovic is No. 2 (behind Nadal) when it comes to winning return points (56%). Djokovic is very fit, but Thiem punishes the ball and has stamina. He's one of the very few pros who can push Djokovic around on the court and dictate to him from the backcourt.

Cambers: Thiem just needs to do what he usually does, and do it perfectly -- serve well, attack with both forehand and backhand and run everything down like his life depends upon it. Djokovic has played within himself so far, now he's going to be tested. This could be epic.

Ford: True self-belief. In his press conference, Thiem recited the Big Three stats we're all familiar with, and noted that he sometimes struggles with Djokovic's flatter shots; but his confidence on clay should help, and he should be well past being overawed by their auras alone. (Djokovic was later asked if Thiem is the Ringo Starr of the group, and wisely dodged the question.)

Which men's final matchup would be most compelling?

Bodo: There's no doubt a Federer win would set up a final with an eerie "I can't believe this is happening" vibe no matter who is on the other side of the net. Nothing could be more compelling than to see Federer poised to win his second title at Roland Garros via a conquest of Nadal. But as matchups go, the gold standard is no longer Federer-Nadal: it's Djokovic-Nadal.

They've already created the richest rivalry in men's tennis, 54 matches with Djokovic leading 28-26. Djokovic would be going for a second non-calendar year Grand Slam (winning four majors in a row, an achievement second only to completing a calendar-year Grand Slam). Nadal appears to be back on his throne and ready to win an unprecedented 12th title. It could not get any better.

Cambers: Federer-Djokovic would be fun. Total contrast of styles, with Federer coming forward, all guns blazing, and Djokovic trying to repel him with the quality of his movement, groundstrokes and passing shots.

Ford: Round 55 of the Djokovic-Nadal heavyweight bout. They're tied at 4-all in Grand Slam finals. With all due respect to Thiem, it would seem fitting for one of the other titans of his era to try and disrupt Nadal's quest for a 12th French Open title and battle through his array of weapons, psychological advantage and what would surely be a partisan crowd.