The roof on Arthur Ashe Stadium is a year from being complete, so it couldn't save Thursday night's U.S. Open women's semifinals from being postponed until Friday.

With the weather forecast calling for rain and thunderstorms through the evening, the USTA on Thursday afternoon postponed those semifinals to the day session Friday.

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Flavia Pennetta will play Simona Halep in the first match followed by Serena Williams against Roberta Vinci. That is the reverse order of the schedule from Thursday night. The matches with start Friday at 11 a.m. in Ashe and ticket holders for Thursday night's matches, Session 21, will be admitted beginning at 10 a.m.

InfographicSerena and the Slams

The men's semifinals will follow, but the stadium will be cleared and Session 22 ticket-holders will be admitted. Novak Djokovic will play defending champion Marin Cilic in the first match scheduled for 5 p.m. Roger Federer then takes on Stan Wawrinka.

With more rain in the forecast this weekend, the Open faces another Monday finish. The roof's importance is at the end of the tournament from the quarterfinals on. It would ensure that the most important matches can be played and help to ensure that players get a day off between the semifinals and final, particularly important for the men because they play five sets.

Rain has played a big part in finally pushing the roof project to completion. Rain had pushed the men's final from Sunday afternoons to Monday for five straight years beginning in 2008. The last two years the USTA scheduled the men's final for Monday and last year pushed the women's final to a Sunday. But in order to get some certainty into the schedule for broadcasters and fans, the roof project had to come to fruition.