Serena Williams' US Open semifinal has been postponed until Friday because of rain in the forecast.
The USTA says both women's semifinals -- No. 1-seeded Williams against unseeded Roberta Vinci of Italy and No. 2 Simona Halep of Romania against No. 26 Flavia Pennetta of Italy -- are being rescheduled for Friday, starting at 11 a.m. ET with the Halep-Pennetta match.
The men's semifinals will be played afterward, beginning at 5 p.m. instead of the originally scheduled 3 p.m. In those matches, No. 1 Novak Djokovic meets defending champion Marin Cilicand No. 2 Roger Federer plays his Swiss Olympic and Davis Cup teammate Stan Wawrinka.
It all makes for a lineup similar to the US Open's old Super Saturday schedule, when the men's semifinals and women's final were played on the next-to-last day of the tournament.
When the USTA announced the rescheduling, there were doubles and juniors matches in progress. But the rain did eventually arrive, wiping out play a little more than an hour before Williams and Vinci were supposed to take the court. It was drizzling at 7 p.m., when that match would have started.
Williams is trying to become the first tennis player since Steffi Graf in 1988 to win all four Grand Slam singles titles -- the Australian Open, French Open, Wimbledon and U.S. Open -- in the same season.
The 33-year-old American has won her first five matches at the US Open and needs two more victories to add that championship to her 2015 collection. One possible issue for Williams is that the final is scheduled for Saturday. Both of her losses in 2015 have come when playing on consecutive days.
The US Open has a long history of scheduling problems due to weather, including five consecutive men's finals postponed from Sunday to Monday from 2008 to 2012.
As part of a broader, $500 million-plus renovation plan for the tournament site, the USTA has been constructing a retractable roof atop the main court at Arthur Ashe Stadium that is planned to be ready for next year's US Open.
For now, a framework of more than 6,500 tons of steel sits atop that stadium, but the work to install the retractable panels will resume after this year's tournament ends.
The Associated Press and ESPN Stats & Information contributed to this report.