* Sharapova upset by Peng
* Djokovic demolishes compatriot
* Dementieva beats Li to reach quarters (adds Djokovic win, Li quote)
BEIJING, Oct 7 (Reuters) - A tired Maria Sharapova bowed out in the third round of the $6.6 million China Open on Wednesday, beaten 6-2 6-4 by China's Peng Shuai to the delight of the crowd at the Olympic tennis centre.
Second seed Novak Djokovic later claimed a place in the men's quarter-finals with a ruthless 6-3 6-0 demolition of fellow Serb Viktor Troicki.
Sharapova, who claimed her first title since returning from a shoulder injury in Tokyo on Saturday, had to come from 5-2 down in the third set to win a three-hour opening match on Tuesday.
There was to be no way back against Peng, however, and the world number 53 converted her third match point to produce an upset for the second evening in a row after her second-round win over holder Jelena Jankovic.
"My energy level was not where it should have been ... there've been occasions where I've been able to fight back, today was just not one of those days unfortunately," Sharapova told reporters.
"She played really solid and really aggressive and did all the right things to win the match."
Peng, who went straight back out to play doubles, must next beat either second seed Serena Williams or Nadia Petrova for a place in the China Open semi-finals for the third time.
Fourth seed Elena Dementieva immediately restored order in the second Russia versus China clash of the evening, rattling through her third-round match against Li Na 6-2 6-0 in just under an hour to move into the quarter-finals.
"That's the worst match I've played in the last six months," said a defensive Li, whose relations with the Chinese media have often been strained.
"It's normal to win and lose, why do you always question my attitude when I lose? Sharapova also lost today, would you say she has psychological problems?"
Svetlana Kuznetsova, the 2006 champion, overcame a misfiring serve to claim her place in the last eight with a 6-3 4-6 6-0 victory over Ukraine's Alona Bondarenko.
The sixth-seeded Russian will face compatriot Anastasia Pavlyuchenkova in the quarter-finals after the teenager beat Canadian Aleksandra Wozniak 6-4 6-4.
BACK IN BLACK
Djokovic was back in his favoured all-black outfit for his encounter with Troicki and displayed no sentiment for his compatriot in a one-sided 61-minute contest.
It would have been an ominous sight for Spain's Fernando Verdasco, who had a much tougher time getting past his own compatriot Juan Carlos Ferrero 7-5 6-4 to set up a last-eight meeting with Djokovic.
Fifth seed Verdasco and fourth seed Nikolay Davydenko are among a handful of players hoping to clinch one of three remaining places at the ATP's season-ending World Tour Finals in London in November.
Russian Davydenko earlier boosted his chances of reaching the season finale for a fifth time with a 6-1 6-0 thrashing of German Philipp Kohlschreiber.
Davydenko will meet Croatian Marin Cilic in the quarter-finals after the eighth seed beat France's Julien Benneteau 6-2 2-6 6-0 earlier on Wednesday. (Additional reporting by Liu Zhen; Editing by Sonia Oxley; To query or comment on this story email email@example.com)