- Serena Williams loses 6-4 6-4 to former world No. 1 Caroline Wozniacki
- The 13-time grand slam winner hit 34 errors during the quarterfinal match
- Wozniacki will play another former world No. 1, Maria Sharapova, in the semis
- Roger Federer's conqueror Andy Roddick loses to 22nd seed Juan Monaco
Serena Williams bemoaned the amount of unforced errors she made after suffering a quarterfinal defeat to fellow former world No. 1 Caroline Wozniacki at the Miami Masters.
The 13-time grand slam winner smashed 34 winners during her clash with the fourth-seeded Dane, but made as many errors in Tuesday's 6-4 6-4 defeat.
The American has won the U.S. hard-court event five times, a feat matched only by Steffi Graf. She is on the comeback trail after a 2011 season which was blighted by injury and illness, including life-threatening blood clots on both lungs.
"I think I just struggled a bit with a lot of errors more than anything," the 30-year-old told the WTA Tour website. "She did well, she moved well, and I just made too many errors. Sometimes you're not playing well. But you have to be able to play better.
"The good thing, though, is I could have played a lot better. I probably played about 20%. It would really suck if I couldn't do any better. I'm just looking forward to the next tournament and I'll try not to dwell on this too much."
Wozniacki, who lost her place at the top of the world rankings to Victoria Azarenka following the Belorussian's triumph at the Australian Open, was pleased to advance after a late rally from Williams.
"There are some players you play who, when you lead 5-1 in the second set, you know that you have pretty much won the match," the 21-year-old said.
"But against someone like Serena, you know you need to fight until the last point. I knew she was going to step up. I could tell she was really motivated that last game."
Wozniacki's reward is a clash with second seed Maria Sharapova, who crushed French Open champion Li Na in straight sets.
The Russian, who has never won the Miami title despite reaching three previous finals, won eight games in a row on her way to a 6-3 6-0 success in just 68 minutes.
The win also ended eighth seed Na's recent superiority over Sharapova, with the Chinese player having won their last four previous meetings in straight sets -- including in the semifinals at Roland Garros last year.
In the men's section, 2010 champion Andy Roddick failed to capitalize on his shock win over 16-time grand slam winner Roger Federer as he stumbled to a 7-5 6-0 defeat to Argentine 22nd seed Juan Monaco.
The 27-year-old Monaco will face U.S. opposition again in the quarterfinals, after eighth Mardy Fish beat Spaniard Nicolas Almagro 6-3 6-7 (2-7) 6-3.
Defending champion and world No. 1 Novak Djokovic beat 17th-ranked Frenchman Richard Gasquet 7-5 6-3 to set up a last-eight clash with Spanish fifth seed David Ferrer, who won 6-3 6-3 against 2009 U.S. Open champion Juan Martin del Potro.
Ferrer's second-seeded compatriot Rafael Nadal proved too good for Japanese rising star Kei Nishikori, winning 6-4 6-4.
The 10-time grand slam winner -- a three-time runner-up in Key Biscayne -- will play sixth seed Jo-Wilfried Tsonga for a spot in the last four. The Frenchman beat German world No.19 Florian Mayer.
Serbia's ninth-ranked Janko Tipsarevic ended the run of unseeded Bulgarian Grigor Dimitrov, winning 7-6 6-2.
He will next play world No. 4 Andy Murray, the 2009 Miami champion, who beat French No. 13 Gilles Simon 6-3 6-4.