* Top seed Azarenka pulls out with inflamed ankle
* Seventh seed Stosur exits due to a calf injury (Adds quotes, detail)
INDIAN WELLS, California, March 14 (Reuters) - In a bizarre coincidence, top seed Victoria Azarenka and seventh-seeded Australian Samantha Stosur withdrew from their quarter-final matches at the BNP Paribas Open on Thursday due to leg injuries.
Australian Open champion Azarenka of Belarus pulled out with an inflamed right ankle after Stosur had exited earlier in the day with a calf injury, handing walkovers for fourth seed Angelique Kerber and Caroline Wozniacki into the semi-finals.
Azarenka, who demolished Russian Maria Sharapova 6-2 6-3 in last year's final, had complained about her ankle earlier in the week and she limped as she made her way toward the players' dining area after her warm-up for the match.
The Belarusian's premature departure gifted eighth-seeded Dane Wozniacki, the 2011 champion here, a place in the last four for a third time at the Indian Wells Tennis Garden.
"I tested it out as much as possible," a visibly distressed Azarenka told reporters. "I tried absolutely everything I could to do, but I have been advised by the doctor, by my own team, that it's just a very, very high risk already.
"There is a big inflammation which keeps bothering one part or another," said the Belarusian, who had an MRI (Magnetic Resonance Imaging) on her ankle on Wednesday.
"It cannot get better with me playing on it, especially playing the last three matches on it. I'm still looking forward to next week (the WTA event in Miami) to see if I can recover and take it from there."
Azarenka was especially frustrated after making a fast start to the year, successfully defending her Australian Open crown in January before going on to win last month's Qatar Open by outclassing Serena Williams 7-6 2-6 6-3 in the final.
"I'm extremely disappointed because I feel I'm playing really well right now," said the 23-year-old who is unbeaten after 17 matches this season, though she also conceded a walkover to Williams in Brisbane.
"I was so excited to be playing here. I love this tournament. But I have to think a little bit in the long-term. I'm going to be back here next year to play, and I'll be as excited again."
Earlier in the day, Stosur pulled out after injuring her right calf on Tuesday while serving for the match during her 4-6 6-2 6-3 victory over Germany's Mona Barthel in the fourth round.
"I don't know if you can get any more unlucky than that," Stosur said in a statement. "I think we only played another four points in that match. I felt something go in my calf and it was hurting a fair bit.
"Yesterday I took the day off practice and only did treatment, probably iced it about 10 times. This morning I thought I would give it a shot and tried to go out and warm up.
"After about five minutes, I knew that there was no chance I could play unfortunately. I think this is only the second time I've ever pulled out of a match in my career."
Kerber, who won her first WTA singles title in Paris last year, said she would make the most of an extra rest day to prepare for semi-final match against Wozniacki.
"For sure it's better when you play and win the match and go to the semis," the 25-year-old German said. "My rhythm now it's a little bit different than after a match. But I have another day off to get ready and try to be fit tomorrow."
The other semi-final in the elite WTA event at Indian Wells will be contested between second-seeded Russian Maria Sharapova and her compatriot Maria Kirilenko.
Sharapova, the 2006 champion here, advanced with a 7-6 6-2 victory over sixth-seeded Italian Sara Errani on Wednesday while Kirilenko booked her place in the last four by upsetting fifth-seeded Czech Petra Kvitova 4-6 6-4 6-3. (Reporting by Mark Lamport-Stokes; Editing by Frank Pingue)