You are not permitted to download, save or email this image. Visit image gallery to purchase the image.
It keeps alive her latest bid for a record-tying 24th Grand Slam singles title.
Williams, a six-times champion in New York, struggled with Pironkova's slice forehand early and was at risk of her earliest US Open exit in 14 years until she used her experience to find a way back.
For Pironkova, who dropped to 0-5 in career head-to-head meetings with Williams, the loss marked the end of a surprise New York run given this was her first professional tournament since Wimbledon in 2017.
Up next for the 38-year-old Williams, who has been pushed to three sets in her last three victories, will be either Belgian 16th seed Elise Mertens or former world number one Victoria Azarenka of Belarus.