US Open organisers are preparing for major weather disruption as Britain's Andy Murray waits to take on world number one Rafael Nadal on Saturday.
The remnants of tropical storm Hanna are forecast to hit the New York area, with heavy downpours expected.
It is understood officials are prepared for play to stretch on until Monday.
The men's semifinals, including Murray's clash with Nadal, are due to start at 1500 GMT, with the women's final from 2300 GMT on Sunday.
The US Open is unique among the four Grand Slams in holding both men's semifinals and the women's final on the same day, dubbed "Super Saturday".
However, tournament officials and the United States Tennis Association have issued an inclement weather warning to ticket holders due to attend.
Murray goes up against Nadal for the sixth time in his career, and the second Grand Slam tournament in a row.
The 21-year-old Briton was soundly beaten at Wimbledon earlier this year, making the head-to-head record 5-0 in the Spaniard's favour.
However, Murray ran Nadal very close on the Australian Open hard-courts last year, taking a two sets to one lead before wilting in a five-set epic.
In their four meetings since, he has not won a set but their encounters indoors in Madrid last year and at the Canada Masters this season were closely fought.
"The one thing that I need to improve is the return," said Murray after his quarterfinal win over Argentine Juan Martin del Potro.
"The first couple of times I played him I returned very well. The last two or three matches I didn't return well at all. That's the only thing that I need to do.
"If I take the match at Wimbledon away, the times I played him on hard courts, I've had quite a few close sets, really tough matches.
"It comes down to who returns better and who gets the breaks of serve. Normally that's the best part of my game. The last few times I've played him, I've not done that."
Like Murray, Nadal will get his first taste of "Super Saturday" after beating Mardy Fish to reach the US Open semifinals for the first time.
The 22-year-old French Open and Wimbledon champion has dropped a set in each of his last two matches on a surface he has struggled with in the past.
But after losing just one match since early May and collecting six trophies, Nadal will be expected to get the better of Murray.
The top seed knows he will have to be at his best.
"Andy is playing very well and having a very good summer so it's going to be a very tough match," he said.
"I know how good Andy is and how good he can play. I know the only way for me to win is play my best tennis.
"I played one of my best matches against him at Wimbledon. I beat him in the semifinals in Toronto too, but it was a very close match and I played well."
The other men's semifinal sees four-times defending champion Roger Federer face world number three Novak Djokovic in a repeat of last year's final.
Federer is through to his 18th straight semifinal, but defeat will leave him without a Grand Slam title to his name for the first time in more than five years.
Djokovic, who beat Federer in the last four on his way to the Australian Open title in January, can push the Swiss down to third in the world rankings if he goes on to win the title.