Stephens lost 6-2, 6-3 to Anastasija Sevastova, the No 19 seed, of Latvia, in the quarter-finals. It's the first time he won the match after that kind of start.
Isner - trying to become the first American man to reach the semis here since Andy Roddick in 2006 - took the first set in a tiebreaker.
NEW YORK-"It's going to be stuck in my mind forever", Dominic Thiem said when his epic, five-set quarterfinal loss to Rafael Nadal was over.
Cilic appeared to be coasting after taking the first set and opening a 4-2 lead in the second. In fact, Nadal never faced match point, and Thiem faced only one all night.
Nishikori, part of a history-making double act with Naomi Osaka in NY as the first Japanese man and woman to make the semi-finals of the same Slam, missed the 2017 US Open with a wrist injury.
Nadal is bidding for a fourth U.S. Open title and 18th Grand Slam trophy overall. Right after the third set tiebreak, the crowd was in for a fourth set tiebreak and this time Thiem came out triumphant.
The victor of this match faces 2009 champion Juan Martin del Potro in the semifinals.
Serena Williams will be one of the final four women standing at the 2018 US Open. However, he then fought back, like Nadal always does, to take the next two sets. "It was a tough battle and it will give me good confidence, even though I don't remember a lot of it", said the 28-year-old after getting revenge on Cilic in the quarter-finals.
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And while Isner's serve was significantly down on pace, del Potro was constantly applying pressure to the strongest part of the American's game. "We both felt exhausted in the end, we both fought all the match, and I think I survived with my serves in a couple of important moments of the match, and that was the only key". "When you're winning a tournament, you think that's normal".
"He has a great two-handed backhand, great footwork and is one of the quickest players on tour".
What has to be done is keeping her game plan and strategies in tact, practicing a bit of change ups and staying positive.
Gauff received a wild card this year into the qualifiers for the main draw at Flushing Meadows, but lost 6-4, 6-1 to Heather Watson of England.
She will now face Anastasija Sevastova in the quarterfinals on Tuesday.
He added "I'm so happy to reach another semi-final in my favourite tournament". It showed on the court, with her giving up on drop shots and bending over to lean on her thighs during brief breaks between serves.
The 20-year-old Osaka is also the youngest semifinalist at the U.S. Open since Caroline Wozniacki in 2010. "But I don't play drop shots when I see the player struggling or bending over".
Sportsmail understands the likely outcome will be a "new day" policy, which in the same circumstances will allow matches to finish outdoors rather than with the roof still in place.
The defeats of Stephens and Pliskova completed the exodus of top 10 seeds, although Pliskova noted that Williams' 17th seeding - nine spots above her world ranking as she continues her post-baby comeback - was not reflective of her true abilities.