* Tired Kvitova crashes out in quarter-finals
* Serena romps into final four
* Radwanska has troubles with serve (Updates after Serena victory, adds quotes)
By Matt Cronin
TORONTO, Aug 9 (Reuters) - Defending champion Petra Kvitova suffered a shock exit when unseeded Romanian Sorana Cirstea beat the Czech 4-6 7-5 6-2 on Friday to move in the semi-finals of the Rogers Cup.
Cirstea was joined by top seed Serena Williams, who thumped Magdalena Rybarikova 6-1 6-1 and set up a semi-final clash with third seed Agnieszka Radwanska who overcame her own ineffective serving to beat Italy's Sara Errani 7-6(1) 7-5.
Fourth seed Li Na of China also advanced with a 7-6(1) 6-2 win over Dominika Cibulkova of Slovakia and will meet Cirstea in the last four.
After playing respectably in the first set, 2011 Wimbledon champion Kvitova could not keep the ball in the court. She doubled faulted on break points in six different games and ended the game with just 12 winners and 55 unforced errors.
Kvitova said that after winning her third round match on Thursday that she could not get to sleep and therefore lacked energy. By the second set, she looked lackadaisical.
"The serve was really bad after this, and I didn't find energy from my legs," she told reporters. "So that's why it looks that bad."
The sixth seeded Czech added that fatigue contributed to her 10 double faults overall with the problem with her serve initially physical before it became a mental one.
Williams, who has only lost 10 games in three matches this week overpowered Rybarikova with big serves and lethal returns.
"I'm definitely feeling pretty good, playing much better than I have in the past month," said Williams.
Radwanska and the fifth seeded Errani embarked on a marathon quarter-final reminiscent of their three-hour clash at the WTA Championships in Istanbul.
They played much the same style of match in Toronto, with both steady off the ground and neither able to dictate with their service game with 11 successive breaks of serve in the first set alone.
"For both of us the serves is never the key, that's why we are always playing long rallies and matches," Radwanska said.
The Pole decided to hit with more power towards the end of the first set, easily taking the tie-break 7-1 with some hard and deep shots.
The pair traded breaks to 5-5 in the second set, but then Radwanska broke Errani for the eighth time of the match to go ahead 6-5 then served out with four winners including an ace.
"I was trying to focus really hard in the last few points because I knew it could really turn around for her," said Radwanska.
"I was looking at the clock and saw two hours already and I said 'Enough I have to end this now'."
Li tossed away her strategy of playing a steady game against the hard hitting Cibulkova after the first set and began to go for her shots more.
"(At the) beginning of the match I was feeling more control the ball, but that was totally wrong because she's the hitter," said Li, who reached last year's final.
"(In the) second set just try to continue to hit the ball." (Editing by Greg Stutchbury)