It was the battle of the seeds as Petra Kvitova overcame Agnieszka Radwanska in Thursday's quarter-final clash at the Connecticut Open.
Defending champion Kvitova started slow but eventually found her groove in a 7-5 6-4 win over seventh-seeded Pole Radwanska at the WTA Tour event in New Haven.
Czech second seed Kvitova rallied from 5-3 down in the opening set and then regrouped after dropping an early break in the second as she set up a semi-final showdown with four-time champion Caroline Wozniacki.
"It wasn't only about the serve today," said Kvitova, who won 75 per cent of her first-service points after one hour, 41 minutes on court.
"She was returning really well. It was a good match from both of us and I'm happy that I'm in the semi-finals."
Former world number one and third seed Wozniacki made light work of Caroline Garcia, dispatching of the Frenchwoman 6-3 6-0 in the final match on Thursday.
Wozniacki - who was drug tested earlier in the day - survived a marathon contest and three match points to reach the quarters at the expense of Roberta Vinci but she had no such trouble against Garcia 24 hours later.
Garcia broke early in the match, however, Wozniacki won 11 of the next 13 games in a comprehensive display.
Fourth seed Lucie Safarova also progressed to the final four via a 6-7 (4-7) 6-4 7-5 victory over Dominika Cibulkova.
Safarova came from a set down and required four match points as she reached the semis.
Next up for Safarova is lucky loser Lesia Tsurenko, who stunned fifth seed Karolina Pliskova 6-2 6-2.