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Rain at the U.S. Open Thursday kept workers busy as they dried the court at Louis Armstrong Stadium.Credit Kirsten Luce for The New York Times

The new retractable roof at the United States Open will not be completed until next year, and so rain still has the power to change the schedule in Arthur Ashe Stadium.

With the forecast calling for rain Thursday, tournament organizers decided to postpone the women’s semifinals from Thursday’s evening session to Friday morning, with play scheduled to begin at 11 a.m.

To accommodate that shift, the start time of the men’s semifinals Friday has been pushed back two hours — to 5 p.m.

The postponement will take Serena Williams’s quest for a Grand Slam out of prime time on the East Coast. She will face Roberta Vinci, an unseeded Italian 32-year-old, in the second semifinal Friday afternoon.

No. 2 seed Simona Halep will play No. 26 Flavia Pennetta at 11 a.m., beginning an extraordinary day of semifinals.

The delay also creates a possibly challenging situation for the 33-year-old Williams. If she beats Vinci, she will play the final, scheduled for Saturday afternoon, without a day of rest.

In 2011, weather delays forced the women’s semifinals and final to be contested on consecutive days, and Williams played one of her least convincing major finals, losing to Samantha Stosur, 6-2, 6-3.

But Williams has won plenty of titles on the regular tour without a day’s rest before the final, and she has overcome every obstacle so far in the four major tournaments in 2015: winning the Australian Open, the French Open and Wimbledon and her first five matches at the U.S. Open.

She is only two victories from finishing off the first Grand Slam since Steffi Graf’s in 1988. Williams has a combined record of 17-1 against the three other players still in contention, with the only loss coming against Halep in the round-robin phase of last year’s WTA Finals in Singapore.

The postponement might have come as a relief to Halep and Pennetta. They won grueling three-set quarterfinals Wednesday afternoon in hot, humid conditions. Pennetta defeated the two-time Wimbledon champion Petra Kvitova; Halep defeated the former No. 1 Victoria Azarenka.

2015 U.S. Open