MADRID (AFP) - Serena Williams extended her unbeaten record this season to 24-0 as she swept aside Spain's Carla Suarez Navarro 6-1, 6-3 to reach the semi-finals of the Madrid Open on Thursday.

Williams had to save three match points in overcoming two-time Australian Open champion Victoria Azarenka on Wednesday, but had no such trouble as she maintained her record of never having dropped a set against Navarro in their six meetings.

The 19-time Grand Slam champion complained that she had been far from ready for the clay court swing when struggling to victory over Italy's Sara Errani in Fed Cup action last month.

However, Williams now believes that was the perfect preparation as she builds up for the French Open later this month.

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

The 19-time Grand Slam champion will face either fourth seed Petra Kvitova or Irina-Camelia Begu in Saturday's semi-finals.