Serena Williams reached the Auckland Classic semi-finals with a straight-set victory over Germany’s Laura Siegemund.

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

Williams, whose last singles title came at the Australian Open in 2017, will play either Amanda Anisimova or Eugenie Bouchard in the last four.

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.

• Live scores, schedule and results
• World number one Barty beaten in Brisbane
• Djokovic puts Serbia into ATP Cup semis

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.