US Open 2015: Simona Halep Sets Up Semi-Final Clash Against Flavia Pennetta

Romanian second seed Simona Halep and Italy's Flavia Pennetta advanced to a US Open semi-final matchup by ousting two-time Grand Slam champions Victoria Azarenka and Petra Kvitova on Wednesday. (Click here for latest Tennis stories)

And now the only obstacles to Serena Williams completing a calendar Grand Slam are players all seeking a first major title.

Halep reached her first US Open semi-final Wednesday by defeating two-time Australian Open champion Azarenka 6-3, 4-6, 6-4.

"I don't know where I found the power to win this match," Halep said. "I'm really happy because I really wanted to go to my first semi-final here."

Italy's 26th-ranked Pennetta upset Czech fifth seed Kvitova, a two-time Wimbledon winner, by 4-6 6-4 6-2 in the first quarter-final at Arthur Ashe Stadium.

"I'm really happy," Pennetta said. "It's something incredible. Before the tournament I never think to be so far in the tournament, so it's something special. It's something amazing for me in this moment."

Pennetta has a 3-1 career edge in matches against Halep.

"I know I have to fight for every ball, to play every ball like the last one," Halep said of playing Pennetta. "I know she is a tough player but I can be strong one more day."

Thursday's other semi-final pits Williams against 43rd-ranked Italian Roberta Vinci, who has not won a set in four matches against the 33-year-old American superstar.

Williams, who holds all four major titles, needs only two wins to complete the first calendar Grand Slam since Steffi Graf in 1988 and win her 22nd Grand Slam title, matching Graf's record for Open Era singles crown.

Williams, seeking her seventh US Open crown, has a head-to-head edge on her possible finals foes -- 7-0 over Pennetta and 6-1 over Halep.

Azarenka, who lost to Williams in the 2012 and 2013 US Open finals, made 42 unforced errors to only 19 by Halep and the 20th seed from Belarus had 38 winners, two fewer than Halep.

Azarenka, 26, beat Halep, 23, twice in 2012 in their only prior matchups but Halep has won three titles this year while Azarenka has been nagged by a thigh injury.

Halep, the first Romanian in the US Open last four since Irina Spirlea in 1997, returned from a third-set rain break and blasted an ace to level at 2-2.

Azarenka sent a backhand wide to hand Halep a break and a 4-3 lead and they held serve to the end, which came when Azarenka sent a forehand long after two hours and 40 minutes.

Pennetta matched her best Grand Slam run from the 2013 US Open semi-finals, where she lost to Azarenka.

"I'm really happy right now," Pennetta said. "I was just trying to fight every ball, running and push everything I can.

"I didn't think to get through this match. In the second set I was playing and playing -- it was unbelievable."

Kvitova's ouster removed a potential threat to Williams who delivered one of only two Williams losses this year when they met in May on Madrid clay.

Kvitova, the first Czech woman in the US Open's last eight since Daja Bedanova in 2001, made 60 unforced errors to only 16 for Pennetta.

The fifth seed also had nine double faults against only four aces in hot conditions at Arthur Ashe Stadium.

"It's not just the heat," Pennetta said. "It's the tension for the match, for what you think, what you have to be, what you want to be, have to do -- it's everything in your mind. You have to take it out and just play."