Maria Sharapova's effort to avenge her Wimbledon final loss to Petra Kvitova was brought to an abrupt end on Thursday after she twisted an ankle in the quarter-finals of the Pan Pacific Open.
The Russian world number two was serving 40-15 up at 3-4 in a well poised first set when she twisted her left ankle on landing after a serve.
Sharapova received court-side treatment but decided to retire soon afterwards, giving the Czech a free ticket to the semi-finals.
Kvitova, who claimed her first Grand Slam title beating Sharapova in June's Wimbledon final, will play Vera Zvonareva, in the semis after the Russian beat compatriot Maria Kirilenko 6-3, 6-3 earlier in the day.
"It always feels great to be in a semi-final. I'm excited about it. It is always nice to go a little bit further than the previous year. I'm looking forward to my semi-final match," said Zvonareva.
Zvonareva, who stood firm in her game while breaking Kirilenko three times, stretched her winning streak against her rival to six in a row without a loss.
"I have no secret," said the fourth seed. "I'm just trying to play my game, trying to find some chances that can cause her discomfort and trying to execute the way I can. So far it's been working good for me."
Third seed Victoria Azarenka of Belarus produced a storming finish by taking the last nine games to beat Marion Bartoli of France 7-5, 6-0 for a place in the other semi-final.
"It was a battle in the first set. Second set I stepped up my level and played a more aggressive style of game. I'm really happy to be in the semis after a little bit of a break as it can sometimes be difficult to get the momentum going," said Azarenka.