Kvitova didn't put a foot wrong in a 6-0, 6-1 semi-final victory over her Czech FedCup teammate Klara Zakopalova. Halep defeated fourth-seeded Caroline Wozniacki -- a former world number one and four-time winner of this event -- 6-2, 7-5.
Errani, ranked No. 5 in the world, became the fifth of the tournament's eight seeds to lose in the first two rounds of the final WTA tuneup before next week's U.S. Open.
Wozniacki advanced to the second round when Peng Shuai was forced to retire while No. 6-seeded Stephens beat Anna Schmiedlova of Slovakia 6-4, 6-4 and will play German Julia Goerges next.
Sabine Lisicki, who lost to Marion Bartoli in the final of Wimbledon, defeated Kristina Mladenovic of France 7-5, 6-1 in the first round.
Petra Kvitova of Czech Republic rose to fifth in the new WTA rankings published Sunday after winning the New Haven Open.
Top-seeded Agnieszka Radwanska bowed out of her opening match at the New Haven Open Tuesday, retiring in the second set against qualifier Olga Govortsova with a sore shoulder.
China's Li Na moved on to the fourth round of the Rome Masters on Wednesday as third seed Agnieszka Radwanska lost to only her third different opponent of the season.
The quarter-final matches of the Japan Women's Open tennis tournament were postponed until Saturday due to steady rain, organisers said on Friday.
A little rain couldn't stop top-ranked Caroline Wozniacki from winning her fourth consecutive New Haven Open title in defeating Czech qualifier Petra Cetkovska 6-4, 6-1 on Saturday.
Three-time defending champion and World No. 1 Caroline Wozniacki won her 16th match in a row over four years at the WTA New Haven Open on Friday to reach the final of the US Open tuneup.
French Open champion Li Na of China rallied from a 3-1 deficit in the opening set to beat Russian Maria Kirilenko 6-4, 7-6 (7/4) in the second round of the New Haven Open.
World No. 1 Caroline Wozniacki won for the first time in over a month, convincingly beating Polona Hercog in the New Haven Open, with Tuesday's play affected by an earthquake.