The 23-time Grand Slam singles champion beat 73rd-ranked Siegemund 6-4 6-3.

Williams, whose last singles title came at the 2017 Australian Open, will play fellow American Amanda Anisimova after she beat Eugenie Bouchard 6-2 3-6 6-4.

Caroline Wozniacki reached the semi-finals with a 6-1 6-4 victory over the 2019 champion, German fourth seed Julia Gorges.

The Dane, who will retire after the Australian Open in Melbourne, will next play unseeded American Jessica Pegula.

Wozniacki and Williams will play their doubles semi-final against Belgian pair Kirsten Flipkens and Alison van Uytvanck later on Friday.

In Brisbane, world number four Naomi Osaka beat Kiki Bertens to reach the semi-finals and extend her winning streak to 14 matches.

Australian Open champion Osaka defeated Bertens 6-3 3-6 6-3 and will play either Czech Karolina Pliskova or American Alison Riske.

Petra Kvitova, who lost to Osaka in the Melbourne final last year, also reached the last four in Brisbane.

The Czech seventh seed defeated Jennifer Brady 6-4 6-2 to set up a meeting with Madison Keys, who beat fellow American Danielle Collins 6-4 6-1.