To many, Serena Williams has already overcome her toughest hurdle in pursuit of the final leg of a historic calendar-year Grand Slam. But, while sister Venus may have been ousted, two hurdles remain before she can hold the U.S. Open trophy aloft and become the first woman in 27 years to take a clean sweep of Major titles in a single year. On Thursday, surprise package Roberta Vinci will stand in her way in the semifinals at Flushing Meadows.

After prevailing in three sets under the lights and a packed crowd on Arthur Ashe Stadium, Serena described her latest victim, the woman she learned the game with in the more humble surroundings of the public courts of Compton, California, as “the toughest player I’ve ever played in my life.” And she was certainly given a test by Venus on Tuesday, losing the second set 6-1 as her elder sister produced some fierce hitting. But Serena again delivered when it mattered, closing out the match 6-3 in the final set to extend her record in three-set matches this year in Grand Slams to 11-0.

Her next opponent will be a very different challenge. A 32-year-old from Italy, Vinci is very much a late bloomer. Her biggest success in her career has come in doubles, where she won her first Grand Slam title in 2012 alongside compatriot Sara Errani before last year they become just the fifth pair in tennis history to complete the Career Grand Slam. Having only reached her first singles quarterfinal at a Major at the age of 29, on Thursday she will be competing in her first ever semifinal.

Her style also marks her out as a trend breaker, playing as she does with a one-handed backhand and a variety eschewing the power-centric approach ushered in by the Williams sisters. Those factors explain why Serena is grateful for having recent experience taking on Vinci in Toronto last month.

“I played her in Canada,” she said. “She played me really tough, and I didn't really expect that. So I'm not going to underestimate her. She played really well. She's not in the semifinals of a Grand Slam for no reason. She knows what to do and she knows what to play. I think it was really good. Again, I just think it was great that I played her because I kind of know what to expect, and I'll be more ready for it this time.

“[She’s] definitely a little bit more old school, but also a really great matchup, because it's fun to see people that can still come to the net and still hit slice and still hit one-handers. It's different. It's good for tennis.”

Still, Williams came through that encounter in straight sets, as she has each of their four meetings. And the unseeded Vinci will certainly be encountering a huge increase in quality on the opposite side of the net when they meet again in Thursday’s evening session. Fortune has shined on her in a route through to the last four that has yet to see her face a single seed. World No. 3 Maria Sharapova withdrew before the tournament, Ana Ivanovic and Carla Suarez Navarro were upset in the opening round, before 25th seed Eugenie Bouchard was forced to withdraw ahead of their fourth-round match after a locker-room fall.

Ranked No. 43 in the world, Vinci showed plenty of resolve, however, to battle past Kristina Mladenovic in a grueling quarterfinal played in intense heat on Tuesday. Her task now is to stop Serena’s date with history in Saturday’s final.

Start time: 7 p.m. EDT

TV channel: ESPN

Live stream: ESPN3, Watch ESPN

Update (3:55 p.m. EDT): Because of rain, the women's semifinals have been rescheduled for Friday. Williams vs. Vinci will follow Simona Halep against Flavia Pennetta, which is set to begin at 11 a.m. EDT.