Madrid, Spain (SportsNetwork.com) - Two-time champion Serena Williams and reigning titlist Maria Sharapova were a pair of quarterfinal winners Thursday at the Madrid Open tennis event.

The world No. 1 Williams whipped 10th-seeded local favorite Carla Suarez Navarro 6-1, 6-3 on the red clay at the Magic Box.

The 33-year-old Williams, who captured back-to-back Madrid titles in 2012 and 2013, still has yet to lose this year, going 24-0, and has won her last 27 matches overall dating back to November.

This is the second-longest winning streak of her career. She won 34 in a row two years ago.

"I definitely didn't have an easy draw, and coming through so far has been really good," said Williams.

"Just because I'm doing good doesn't mean my preparation was better. I didn't have the greatest preparation that I would've liked to. I would've liked to spend so many more hours on the court just sliding and stuff.

"But I'm telling you, Fed Cup really helped me out. It helped propel me to the extra preparation that I needed."

Williams is playing in only her third WTA tournament since clinching her sixth Australian Open title and 19th Grand Slam singles championship back in January.

Her semifinal opponent on Friday will be fourth-seeded Wimbledon champ and 2011 Madrid titlist Petra Kvitova, who handled Romanian Irina-Camelia Begu 7-5, 6-3.

Williams is a perfect 5-0 lifetime versus Kvitova. The two women were scheduled to meet in a quarterfinal here a year ago, but Williams pulled out of the match due to a thigh injury.

Meanwhile, the third-seeded Sharapova, who's chasing an 11th career title on clay, held off fifth-seeded fellow former world No. 1 Caroline Wozniacki 6-1, 3-6, 6-3 on Day 6 Madrid.

Sharapova is now 6-4 lifetime against Wozniacki, who had won their previous two meetings.

The French Open champion Sharapova topped Romanian Simona Halep in last year's final in the Spanish capital and was the 2013 runner-up here to Williams.

Wozniacki was a Madrid runner-up in 2009.

Sharapova's semifinal opponent will be unseeded fellow Russian Svetlana Kuznetsova, who outlasted 13th-seeded Czech Lucie Safarova 5-7, 7-6 (7-5), 7-6 (7-3) in a torturous 3-hour, 5-minute affair.