Victoria Azarenka defeats Kim Clijsters in Miami
World number two Kim Clijsters said she lacked "fighting spirit" after she lost to eighth seed Victoria Azarenka in the last eight of the Sony Ericsson Open.
The defending champion, who saved five match points in the previous round against Ana Ivanovic, lost 6-3 6-3.
Azarenka goes on to face Russian Vera Zvonareva in the semi-finals in Miami.
In the men's event, Novak Djokovic improved his 2011 record to 22-0 with a 6-4 6-2 win over Kevin Anderson to set up a semi-final against Mardy Fish.
Djokovic, 23, remains the man to beat after saving five break points and holding serve throughout the match against South African Anderson.
"It was anything but easy," said Serbia's world number two. "The result doesn't show what we had on the court, definitely. I kind of expected him to be aggressive, but he was really going for the shots."
Fish, currently ranked 15th in the world, will overtake Andy Roddick as the number one American after a 7-5 6-2 win over Spain's David Ferrer.
"I don't think I will ever feel I am the top-ranked American," said the 29-year-old Fish. "Andy has had a pretty good career and he has always been top dog in my generation."
Zvonareva reached the last four with a 7-5 6-3 win over Agnieszka Radwanska of Poland.
Radwanska earned herself a set point in the first set but Zvonareva saved it with a timely ace, before the Russian recovered to clinch the set.
The world number three will next face Azarenka after the Belarusian brought an end to Clijsters' campaign, the Belgian running out of steam in her first tournament since suffering a shoulder injury in Indian Wells.
"I think tonight was going to be very tough, obviously, but I just didn't feel good out there," said Clijsters. "Just mentally, physically, I didn't feel right.
"It has been a tough few last days but not in a way that I should not be ready for that. I train hard enough to physically be capable of doing that.
"Mentally I just kind of feel like I didn't have any fighting spirit. It's obviously tough against someone like her.
"Against some lower-ranked players maybe it's possible to get through matches, but against somebody like her, not."
Azarenka said: "Kim had a very tough match yesterday, but with Kim you never know. She's such a good player and such a good fighter, I knew she would fight for it, which you could see at 6-3 5-1, 40-15.
"She went for her shots and stayed in the match until the end. I knew it would be tough and I was ready."