NEW YORK (TIP): 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 a.m. (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.

Another day of tension only heightened the anticipation and drama around Williams and her quest for more tennis history.

The 33-year-old American, who already holds all four major titles, is chasing the first calendar Grand Slam since Steffi Graf in 1988 and trying to match Graf’s Open Era career record of 22 Slam singles titles, two shy of Margaret Court’s all-time record.

“I never really thought I would be in this position where I would even be talking about records, talking about passing Steffi Graf or even mentioning Margaret Court,” Williams said.

“I just grew up trying to be the best that I could and do the best that I could. I worked really hard for everything, so to even be mentioned in that conversation is great.”

Three-time defending champion Williams, who seeks an Open Era-record seventh US Open title, has won all four career meetings against Vinci without dropping a set.

She also is 7-0 against Pennetta and 6-1 against Halep in their career rivalries.

Only five players have completed the calendar-year sweep of the Australian, US and French Opens and Wimbledon –Americans Don Budge in 1938 and Maureen Connolly in 1953, Australians Rod Laver in 1962 and 1969 and Margaret Court in 1970 and Germany‘s Graf in 1988.

“It is important to me,” said Williams, who has kept talk of the calendar Slam at a bare minimum to help cope with the pressure.