"I was serving well and keeping the ball very well... I just felt like John Isner for a little bit there," Radwanska, who is eyeing her third Pan Pacific Open title, said in an on-court interview.
World number four Radwanska will take on former world number one Caroline Wozniacki, who also put in a strong performance on Friday to beat Magda Linette 6-4 6-3.
Naomi Osaka of Japan beat Belarus' Aliaksandra Sasnovich 6-3 7-6(6) to set up a semi-final match against Ukraine's Elina Svitolina, who upset world number three Garbine Muguruza 6-2 4-6 6-3.
The 18-year-old Osaka was 5-0 down in the second set but clawed her way back to reach her first semi-final at the Pan Pacific Open.
Svitolina and Osaka have played twice before, with each winning once.
(Reporting by Shravanth Vijayakumar in Bengaluru; Editing by Toby Davis)
