2014 US Open champion Marin Cilic seemingly struggled for form in the first half of the 2016 ATP World Tour season, however, the Croat is thriving in the second half of the season.
The ninth seed finally defeated world number one and three-time defending champion Novak Djokovic in the quarterfinals at the 15th time of asking.
Cilic is searching for his second Masters 1000 title after he claimed his first at the Western and Southern Open in Cincinnati, ending Andy Murray's hot streak back in August.
Meanwhile, John Isner started the year ranked firmly inside the top 20 but with a failure of defending points, and not being able to string together a consistent amount of results, Isner's ranking has slipped almost out of the top 30.
However, the big-serving American is rediscovering some form as he looks to end his 2016 campaign on a high and claim his first title of the year.
Cilic's route to the semifinals
Cilic has been playing scintillating tennis this week in Paris, and the Croat has yet to surrender a set throughout these Championships.
The ninth seed had the privilege of receiving a first round bye, and in the second round, he was up against his Davis Cup teammate Ivo Karlovic, defeating his compatriot 7-6 (7), 6-2.
In the third round, Cilic sealed his qualification for the ATP World Tour Finals for the second time in his career by defeating 8th seed David Goffin for the first time in four meetings, thus ending the Belgian's chances of qualifying for London.
Cilic was dealt with a tough assignment in the quarterfinals by facing the top seed and three-time defending champion Novak Djokovic who had a commanding lead over the Croat.
However, Cilic finally recorded his first victory over Djokovic at the 15th attempt with a 6-4, 7-6 (2) scoreline, which now means that Andy Murray is in pole position to wrestle away the world number one ranking from the Serb.
Isner's route to the semifinals
The 31-year-old American has been ranked as high as number nine in the world, and usually was consistently ranked in the top 16, meaning that he would be seeded for a Masters 1,000 event.
Unfortunately for Isner, currently ranked at a lowly 27, he is unseeded but has taken advantage of a good draw.
Isner dispatched Mischa Zverev, 7-6 (8), 6-4 in the first round which was a tough match considering how well the German has been playing, and he was two points away from defeating Djokovic in Shanghai.
In the second round, Isner defeated 15th seed and 2012 champion David Ferrer, who has been struggling for form throughout this year, 7-6 (4), 7-6 (4).
The 2011 semifinalist was scheduled to face the reigning US Open champion Stan Wawrinka in the third round, but he faced Wawrinka's conqueror Jan-Lennard Struff and dispatched him in three sets, 6-4, 6-7, 6-3 to reach his first Masters 1,000 quarterfinal this year, and first since last year in Paris when he lost to Ferrer.
Isner defeated his compatriot Jack Sock in an all-American quarterfinal clash with a tight 7-6 (6), 4-6, 6-4 victory.
The two big servers have met on six occasions, and the Croat has a flawless 6-0 head-to-head record in his meetings with Isner.
Not surprisingly all of their encounters have involved at least one tiebreak.
Cilic won their first meeting in the third round of the Australian Open in 2011, recovering from two sets to one down to defeat Isner, 4-6, 6-2, 6-7(5), 7-6(2), 9-7.
The ninth seed won their next three meetings in straight sets in 2012 at the Mutua Madrid Open, third round of the Miami Open in 2013 and in Delray Beach in 2014.
However, their third round meeting at Wimbledon in 2015 was reminiscent of their Australian Open encounter four years prior, as another five set battle ensued with Cilic coming out on top, 7-6(4), 6-7 (6), 6-4, 6-7(4), 12-10.
Cilic was victorious in their most recent meeting in the Davis Cup in straight sets which was a key victory in Croatia's comeback victory over the United States.
Who reaches the final?
Both players are no strangers to reaching the latter stages of Masters 1,000 events as both participants have reached three finals between them with Cilic winning one.
Unfortunately, Isner lost to Roger Federer in the 2012 BNP Paribas Open final in Indian Wells and to Rafael Nadal in Cincinnati in 2013.
However, Cilic will be full of confidence since ending his losing run against Djokovic, and not dropping a set this week, along with having a winning streak points in favor of a Cilic victory.
On the other hand, Isner serves well indoors which he has portrayed throughout this tournament, and the unseeded American can return well in his own right.
This match will be serve dominated but both players will be looking to capitalize on their opponent's frailties if they are struggling on their serve.
Isner's forehand is a dangerous weapon, and Cilic's forehand has been firing on all cylinders too, and another thing which works in Cilic's favor, is that he moves slightly better than the American.
This is the first of two men's singles semifinals match scheduled on Court Central and this one is due to start at not before 2pm local time.
The winner of this match will face fourth seed and 2014 finalist Milos Raonic or second seed Andy Murray in the final, and the latter could be the world number one should he defeated Raonic in a rematch of their Wimbledon final meeting this year.
Coincidentally, Murray defeated Raonic in the semifinals of the Western and Southern Open, and Cilic defeated Murray in the final to claim his Masters 1,000 title. An Omen for Cilic perhaps?
Prediction: Cilic in straight sets