Madrid - Maria Sharapova beat Li Na to book her place in the last four of the Madrid Open as top seed and reigning champion Serena Williams pulled out of the tournament on Friday.
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.
"It happened during my first round match. It started to get better, but most importantly right now, I just need some time to rest and recover.
"It's beyond words. It's so frustrating."
However, the American insisted that she will try to defend her title at the Internazionale BNL D'Italia in Rome next week.
"I am planning to play in Rome. Fortunately, I have a bye next week so will have an extra day of rest.
"For now, I will take it one day at a time and take advantage of some extra rest."
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.
"I will say the match was very close. I think it really could have gone her way today," admitted Sharapova.
"It was just a matter of a few points in the second set and in the third, although I was up a break throughout the third set, you know she's not going to let go. I didn't win the match until the last point was played."
Li looked on course for the semi-finals when she broke to lead 4-3 in the second set.
However, Sharapova broke back immediately and, despite needing four set points, finally forced the match into a deciding set when Li netted at 5-6 down in the tie-break.
There were three breaks of serve to open the third set, but Sharapova took control when she finally consolidated the break at 3-1 and was gifted the final game when Li double faulted on match point.
The four-time Grand Slam champion will face third seed Agnieszka Radwanska or French qualifier Caroline Garcia in the semi-finals.
Also into Saturday's semis is fourth seed Simona Halep as she set up a meeting with Kvitova thanks to a 6-2, 6-2 thrashing of Serbia's Ana Ivanovic.