It left one of the signature races of the games without one of its main characters — a challenge by Liu of defending champion Dayron Robles and U.S.-hurdler Aries Merritt which was to be one of the highlights of the Olympics.
The biggest duel — between 100 champion Usain Bolt and training partner Yohan Blake — continued apace, with both cruising into Wednesday's semifinal of the 200, where Bolt is seeking his second Olympic sprint double.
But Liu's hopes for a second gold are over — for at least another four years.
"The initial assessment of medical experts is a suspected ruptured Achilles tendon," Chinese team leader Feng Shuyong said, adding that "the injury should be quite serious."
Liu won in 2004 at Athens to become the first Chinese athlete to claim an Olympic track and field gold medal, but shocked the home crowd in Beijing four years later when he had to withdraw in his heat without clearing a hurdle because of a right foot problem which later required surgery.
"He's one of the best," Bolt said of Liu. "For him to push himself and come back last year and for this to happen, it's really sad for any athlete."
This time, his coach had already expressed concern that Liu was less than fully fit, but he had still been expected to challenge for gold in London.
Liu had his right Achilles protected with yellow tape when he came into the stadium. While other runners flew over hurdles for practice, he jogged up to the first hurdle and raised his eyebrows.
He produced a wan smile when the stadium speakers announced his name to the cheering. He was talking to himself as he started to settle in the blocks.
Then from the starting gun, it all went wrong for one of the biggest stars in the sport. With his lead leg, he crashed into the first hurdle and tumbled over. He looked up and held his right leg, realizing history has just repeated itself.
Instead of being wheeled off, he hopped on one leg along the track, going over to kiss the last hurdle in his lane 4.
"His spirit is there," Feng said. "Just like the Olympic spirit: winning is not so important but participating is the most important thing."
When he came to the finish line, Balazs Baji of Hungary was awaiting him and raised the arm of the former Olympic champion.
Britain's Andrew Turner and Spain's Jackson Quinonez then led him off to a waiting wheelchair.
"That's two Olympics in a row he's limped off with an Achilles problem — I wouldn't wish that on my worst enemy," Turner said. Liu had a resurgence in form over the past two years, which made him a legitimate gold medal contender again.
Merritt was the top qualifier for Wednesday's semifinals with 13.07. Cuban world-record holder Robles and world champion Jason Richardson of the United States also won their heat.
The 200 had far less drama, even with Bolt back at his showboating best. All business during his heats of the 100 before he won the biggest race of the games ahead of Blake, he was now happily showing off his "To the World" signature move even before he got into the blocks, and chatted with a volunteer taking care of his gear before giving him his cap. He almost jogged home for the easiest of victories.
"It was good. Got to take it easy," Bolt said.
Blake was just as impressive in his race, though much more understated. France's Christopher Lemaitre also advanced with a victory.
If Bolt is going for a sprint double, Tirunesh Dibaba of Ethiopia is seeking a long-distance one and the women's 10,000 champion easily qualified with the top time in the 5,000.
The top six qualifiers in the 5,000 all came from Ethiopia and Kenya, assuring that the East African rivalry will continue through Friday's final.
Injuries took their toll on the morning program.
In the triple jump, the home crowd at the packed 80,000 Olympic Stadium had high hopes for Phillips Idowu, the former world champion and runner-up at the Beijing Olympics.
However, there were fears about a nerve problem in his back, and he produced only the eighth best jump in the first qualifying group, leaving him out of Thursday's final.
"I struggled with some nervy pains and haven't been 100 per cent," Idowu said. "I always think if I could come here and be pain free then I should be able to replicate what I've done in previous years."Suggest a correction