Venus Williams ends Muguruza's tilt at the WTA Finals title
The veteran American saw off the Spaniard in straight sets in Singapore to reach the semifinals for a fifth time in five appearances at the season finale.
Venus Williams moved into the last four of the WTA Finals and potentially two games from returning to the world number one ranking for the first time since 2002 when she knocked out second-ranked Garbiñe Muguruza in straight sets on Thursday.
Williams, 37, defeated Muguruza 7-5, 6-4 in one hour and 41 minutes to become the second semi-finalist out of White Group behind world number three Karolina Pliskova, who finished top.
The seven-time Grand Slam winner moved around the court crisply, showing no ill effects from Tuesday's three-hour marathon against Jelena Ostapenko, as she dumped out Wimbledon champion Muguruza.
"I was happy to win the last point," Williams said. "It is not easy out here, you have to play every point well otherwise you lose the game.
"I had some great coaching coming into the game, I hope we can win some more."
Williams could become number one for first time in 15 years
With the victory, Williams maintains her hopes of returning to world number one for the first time in 15 years by winning the season finale on Sunday.
But it was a tame end for Muguruza, the WTA player of the year who demolished Williams in this year's Wimbledon final, but who faded in Singapore after a strong start.
With both players needing to win to progress, they came out looking sharp but they also struggled to hold serve and Williams broke Muguruza five times to propel her to victory.
The win means Williams, who lifted the trophy in 2008, has never failed to reach the semi-finals in five appearances at the season-ending tournament.