The fifth-seeded Pole defeated the 45-year-old seven-time Grand Slam singles champion 6–2, 6–2 in an hour and 12 minutes, Polish state news agency PAP reported.
Fręch will face Kazakhstan's Elena Rybakina on Friday for a place in the last four.
Fellow Pole Magda Linette, ranked No. 28 in the world, crashed out of the tournament after losing 4-6, 0-6 to Russian Anna Kalinskaya in the second round.
The 2025 Washington Open, also known as the DC Open, runs until Sunday.
(gs)
Source: IAR, PAP