Despite her 58th-ranked world, Lee is a potentially tricky opponent, finishing second with 22 points in the first three games.
The 25-year-old looked confident when he chose to serve for the first time after winning the coin toss, only to have Pegula break it immediately.
Lee made progress after making 10 uncompulsory mistakes, winning just 8% of her second innings in the first set, but Pegula seemed to improve her own game.
As usual, Pegula is also strong at baseline, but also consciously works to move online. Lee, by contrast, tends to stick to a boring effort to keep up with Pegula’s powerful stroke.
“I feel very comfortable today, coming in at the right time, it doesn’t always feel good, but it starts well and I want to execute until the last game,” Pegula said.
“This is part of a game that I can really use as a weapon. Sometimes I get stuck in the back because I’m great there, but we’ve done a lot of work and when I’m done elevated and volleys online, my coach is happy.”
In the quarterfinals, Pegula will face World No. 62 Barbora Krejcikova or fellow countryman Taylor Townsend, who eliminated former French Open champion Jelena Ostapenko and world No. 5 Mirra Andra Andreeva without falling.
