There are four men remaining in the U.S. Open but only one of them is a member of the Big Four.

Roger Federer, Rafael Nadal and Andy Murray are all out — leaving No. 1 seed Novak Djokovic as the lone member of the so-called Big Four remaining in Friday's semifinals.

Djokovic, from Serbia, will meet Frenchman and No. 10 seed Gael Monfils in one Final Four showdown, while No. 3 Stan Wawrinka of Switzerland will face No. 6 Kei Nishikori of Japan in the other.

"I'd say Novak would be the favorite," Murray, the Wimbledon and Olympic champion and world No. 2, said Wednesday after losing to Nishikori in a five-set quarterfinal epic that lasted nearly four hours.

"Kei is obviously capable on this surface of beating the best players and has beaten Novak here, as well. I would put Novak as the favorite, but everyone's got a chance."

This marks the first Grand Slam since 2004 where Federer, Nadal and Murray aren't represented in the semifinals. (Federer didn't play the Open due to knee problems.)

But Djokovic remains alive for his 13th Grand Slam title, which would put him one behind the 14 achieved by Nadal and Pete Sampras, and four back of Federer's all-time mark of 17. He's had a surprisingly easy path to the semifinals because two of his opponents retired during their matches, and a third withdrew before ever playing.

"Grand Slams are valued the most in our sport as tournaments historically, so these are the tournaments where I want to do well and do my best," Djokovic said before the tournament began.

The Serb is 12-0 all-time against Monfils, the flashy Frenchman who appears to be playing a more focused and disciplined brand of tennis these days.

"Gael has been playing very well," Murray said. "I think in their head-to-heads, Novak has had quite a little bit of success."

Wawrinka outlasted 2009 champion and fan favorite Juan Martin del Potro in four sets in a match that ended early Thursday morning.

The only man remaining other than Djokovic who has won Grand Slams (two), Wawrinka, 31, is seeking to become the fifth man in the Open Era to win multiple major titles after age 30.

The Swiss is 3-2 against Nishikori, who reached the 2014 final before losing to Marin Cilic. Nishikori is 5-0 against Top 10 men at the Open, and 3-11 at all the other majors.

"Well, it's going to be interesting for sure," Wawrinka said. "We played many times against each other. He beat me a few years ago here [2014] quarterfinal five sets. He beat me in Toronto; I beat him in Australia. I saw him play [Wednesday]. He was playing really well.

"It's going to be tough match, for sure. We will see also how I'm going to feel physically in two days to get ready for that."

Follow Adam Zagoria on Twitter @AdamZagoria for updates throughout the U.S. Open.