Serena Williams was in dominant form as she beat Maria Sharapova 6-1 6-3 to reach the semi-finals of Bank of The West Classic in Stanford, California.
In losing her sixth match in a row against the American, Sharapova hit 23 unforced errors.
Williams will meet Sabine Lisicki - who beat Poland's Agnieszka Radwanska 7-6 2-6 6-2 - in one of the semi-finals.
In the other last-four tie, third-seed Marion Bartoli from France will take on Slovakia's Dominika Cibulkova.
Williams is playing in only her third tournament since winning Wimbledon in July last year.
She is currently ranked 169th in the world after spending time recovering from foot surgery and a blood clot on her lung.
There was little evidence of her long lay-off, however, as she sent down 17 winners against Sharapova, who lost in this year's Wimbledon final to Petra Kvitova.
"I had a good start," Williams said. "You can't go in against the fifth-ranked player with your mind everywhere. You have to be serious.
"I'm a better player this month than I was last month. I've put in a lot of work and I hope it continues to pay off."
Williams is building up to the US Open in New York at the end of August, a tournament she has won three times.
Sharapova will certainly be hoping to improve her form.
"It certainly wasn't my night," she said. "Serena was serving and hitting so well and I was extremely late in my reactions. I felt sluggish.
"It was a bad day but it's also a reminder that I need to step up.
"When I had chances to change things around I couldn't do it. Her biggest strength is her power and you don't see so much of that in the top 10. It really makes her dangerous."
Standing in Serena's way for a place in the final will be Lisicki. The German is enjoying the best spell of her career, having reached the Wimbledon semi-finals and risen to 26 in the world rankings.
Lisicki, like Williams, has suffered from recent injury troubles, and spent five months out of the sport last year recovering from a serious ankle injury.
She ousted the higher-ranked Radwanska in the quarter-finals, and set her sights on further progress, saying: "I love competing in the big matches. I have nothing to lose. Coming from grass to hardcourt is always a challenge. It comes down to who plays the best that given day.
"I'm much better than I was before the injury. I've had to focus on so many aspects of my game. Last year I couldn't even walk. Now I feel I can rely on my serve when I need it at crucial moments."
Bartoli was given an easier passage to the last four as opponent Ayumi Morita of Japan retired after losing the first set 6-1. The third seed is a former champion at Stanford, and is seeking her second tour title of the year. In June she won the AEGON International at Eastbourne.
Cibulkova will play Bartoli for a place in the final, after the Slovakian eighth seed beat New Zealand qualifier Marina Erakovic 6-1 6-1.