Despite coming into the tournament with doubts following surprise defeats to David Ferrer and Nicolas Almagro in recent weeks, Nadal has looked in fine form in the Spanish capital as he has cruised into the last four without the loss of a set.Maria Sharapova also booked her place in the last four of the Madrid Open with a three-set win over Li Na, but top seed and reigning champion Serena Williams was forced to pull out of the tournament due to a thigh injury.

The 13-time Grand Slam champion will meet compatriot Roberto Bautista Agut or Colombia's Santiago Giraldo on Saturday for a place in the final.

Williams was due to face Petra Kvitova in the quarter-finals but pulled out due to a thigh injury she had been carrying since sweeping aside Swiss teenager Belinda Bencic in the first round on Sunday.

"I have a left thigh injury and, unfortunately, have to withdraw from this year's Madrid Open," said Williams.

However, the American insisted that she will try to defend her title at the Italian Open in Rome next week.

Sharapova has been beaten by Williams in Madrid in the last two years, including in the 2013 final, and the Russian is now the favourite to clinch her first title in the Spanish capital as she edged a thrilling match against second seed Li 2-6, 7-6 (7/5), 6-3.

The four-time Grand Slam champion will face third seed Agnieszka Radwanska or French qualifier Caroline Garcia in the semi-finals, AFP reported.