Swiatek bounced back from a second set slump to win 6-4, 4-6, 6-1 on the hard courts of San Diego on Thursday evening. While the Pole broke late in the opening set to take the lead, Zhang would win the second set and go up 1-0 in the third.
But that's where Swiatek shifted gears, winning six games in a row to secure her 61st victory of the season.
"Qinwen, she has a different game style because she's playing topspin on her forehand and the ball is flying pretty high," the three-time grand slam winner told the press after the match.
"So today I had to adjust to that. But overall, the first matches of any tournament is about adjusting to the conditions we have here."
The win books Swiatek a quarterfinal date with Coco Gauff, a rematch of the French Open final. Unfortunately for the American, she is yet to take a set off Swiatek in their three meetings.
Meanwhile, Pegula defeated Coco Vandeweghe 6-3, 6-1, securing a spot in the last eight in San Diego and - more importantly - a place at the prestigious WTA Finals for the first time in her career.
Pegula joins Swiatek and Ons Jabeur in the season-ending finale, with five qualification spots still up for grabs.
Well that escalated quickly ðº@iga_swiatek recovers in the third set to notch win number 61 this season!— wta (@WTA) October 13, 2022
Faces the winner of Andreescu-Gauff in the #SanDiegoOpen last eight pic.twitter.com/xembNoj5Er