By Kavitha Shastry

We're down to the final matches of this year's U.S. Open, with the four women still in the running for the ladies' singles title facing off in the semifinals Thursday night.

And for Todd Martin, a finalist in New York in 1999 and currently the president of the International Tennis Hall of Fame, it's basically a story of Serena Williams...and everybody else.

"I wouldn't go so far as to say it's Serena's tournament to win or lose, but she has gotten to that stage in the tournament where she's the only one who's ever won a Major before, and she's playing really well," Martin said in an interview on Cheddar Thursday.

"She's reasserted herself as, really, the best player in the game."

Williams, who's going for her seventh U.S. Open title, came into this year's event as the 17th seed but was considered by many to be a favorite to win. So far she's defeated her sister Venus, herself a two-time champion, and eighth seed Karolina Pliskova, the runner-up in 2016.

Williams kicks off Thursday's singles action against Latvia's Anastasija Sevastova, who's playing in her first ever Grand Slam semi. The 19th seed may be the heavy underdog, but she's had a good run in New York, dismissing defending champion Sloane Stephens in straight sets in their quarterfinal.

In the second match-up of the night, 14th-seeded American Madison Keys, who lost to Stephens in last year's final, faces rising star Naomi Osaka of Japan. Keys has won all three of the pair's previous meetings, but Osaka, with her first career title at a tournament in Indian Wells, Calif., earlier this year and a win over Serena in Miami, cannot be counted out.

As for the men, who contest their semifinal matches on Friday, there's a little more star power in the mix.

Three-time champion Rafael Nadal, winner in 2017, is coming off a marathon quarterfinal against Austria's Dominic Thiem and will face Argentina's Juan Martin Del Potro, who earned his first and so far only Major title in New York in 2009.

Martin said this match will be one to watch.

"On paper, you would say Nadal has the edge. He's won so many Majors, he's in good form right now, and he's really and truly the best fighter in our sport," Martin said.

"However, match-up-wise, Del Potro is 6'6", he's playing at a high level. Nadal's lefty game matching up with Del Potro's height, it's neutralized a little bit...I wouldn't be a bit surprised if there was a minor upset with Del Potro winning that match."

The second men's semi though may be less of a challenge. Former world #1 Novak Djokovic, coming off a title at Wimbledon this year, will take on Japan's Kei Nishikori, a finalist in 2014.

Martin said the favorite is clear.

"Djokovic, when he's right in the head and in the body, he's the best player in the world, and I think he's in a pretty good place right now. I expect him to win the tournament, but there's a few more matches left to be played."

For full interview click here.