New York - Two triumphs from completing a calendar Grand Slam, top-ranked defending champion Serena Williams was confronted Thursday at the US Open by an opponent even she cannot defeat - rain.
US Tennis Association officials postponed the two scheduled women's semi-final matches to Friday due to forecasts of rain Thursday lasting through the night.
Top-ranked defending champion Williams now will play Italy's 43-ranked Roberta Vinci in Friday's second semi-final after Romania's second-ranked Simona Halep faces Italian 26th seed Flavia Pennetta in the opener at Arthur Ashe Stadium starting at 11 am (1500 GMT).
The winners will meet in Saturday's final.
Men's semi-finals on Friday were delayed two hours to 5 p.m. (2100 GMT) to allow for the extra matches.
The delays come in the final uncovered year for Arthur Ashe Stadium, which starting in 2016 will have a retractable roof to avoid any bad weather postponements.
A supporting superstructure is in place around the venue now but there was not enough time to secure the roof in place before the tournament.