The 2015 US Open Tennis women's competition is now down to four players and all of them will look to move one step closer to winning the final Grand Slam title of the year on Thursday.

Serena Williams of United States, the player tagged as heavy favorite even before the tournament started, will go up against unseeded Roberta Vinci of Italy in the first semis match of the day.

Williams advanced to the semis via a three-setter victory over her older sister and 23rd seed Venus Williams, while Vinci made it into the final four also with a three-setter win against Kristina Mladenovic.

Williams, who is looking to sweep all four Grand Slam competitions this year and also grab her seventh US Open title, is a heavy favorite heading into the match.

The top-seeded Williams, a 21-time Grand Slam champion, has swept all four previous meetings with Vinci, including their quarterfinal encounter in Toronto earlier this year. However, Williams said that there is no reason to underestimate Vinci in their upcoming semis match.

"I'm not going to underestimate her," Williams said. "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."

On the other hand, second seed Simona Halep will also look to advance to a Grand Slam final for the first time this year when she collides with 26th seed Flavia Pennetta in the second semis match of the day.

Halep booked a semis seat after a 6-3, 4-6, 6-4 win against 20th seed Victoria Azarenka of Belarus, while Pennetta pulled of a 4-6, 6-4, 6-2 upset victory over fifth seed Petra Kvitova in the quarterfinal.

Halep and Pennetta will meet for the fifth time in their respective careers, and the latter holds a 3-1 head-to-head advantage. Halep said she is familiar with Pennetta and she admits she will have a tough night on Thursday.

"It's tough. I know her," Halep said. "She's very smart on court. She's fighting. I know that she never gives up. During the match [Wednesday] I saw a little bit, because I was warming up, and I know that she has a great backhand. I have just to play smarter than usual."

The Williams-Vinci match will start at 7 p.m. ET at Arthur Ashe Stadium, which will be followed by the Halep-Pennetta showdown. Fans can watch the matches on ESPN and online streaming on US Open Live and WatchESPN.