Serena Williams thrashed Maria Sharapova to reach the semi-finals of the Madrid Open on Friday, while Victoria Azarenka is also through.

The American triumphed 6-1 6-3 to claim her 11th successive victory on clay.

Sharapova, a recent winner in Stuttgart, was again beset by problems with her serve. She produced eight double faults and was shown no mercy by Williams.

The new blue clay on show in the Spanish capital is playing fast, something which would appear to aid Williams' power game.

She has already won on clay in Charleston this season - another venue known for its court speed - but whether Williams will be able to continue her impressive form on the slower courts of Rome and Roland Garros remains to be seen.

Williams, unbeaten in 11 matches on clay this year, will face qualifier Lucie Hradecka who stunned US Open winner Sam Stosur as her dream week goes on.

She toppled Wimbledon champion Petra Kvitova earlier in the tournament and scrapped her way to a 7-6 (10/8) 7-6 (8/6) triumph on Friday.

Earlier in the day, Azarenka fought back from a set down to reach the last four.

The world number one found herself in trouble against reigning French Open champion Li Na but recovered to post a 3-6 6-3 6-3 success.

Li started like a train and when she broke serve in the opening game of the second set, a shock looked on the cards.

However, Azarenka retrieved the break immediately and pushed on in the second set to level the match.

The momentum was well and truly with the Belarusian entering the decider and she duly raced 4-0 in front.

Li found a second wind though, recovering to 4-3 and then threatened to break Azarenka again to level the set.

However, Azarenka held her nerve, her serve and closed the match out in the following game.

She will now meet Agnieszka Radwanska, who saw off Varvara Lepchenko 6-4 6-4.