Liu, Bolt set to light up Olympics track

Chinese track idol Liu Xiang returns to Olympic duty on Tuesday, four years after his Beijing dream was shattered, while 100m champion Usain Bolt begins the defence of his 200m title. Liu, 29, didn't tackle one hurdle in Beijing's Bird's Nest Stadium in 2008, pulling out because of an Achilles problem, and the 2004 gold medallist has only just returned to his best. In Tuesday's heats, he will have an extra motivation to snatch back his crown as he renews his rivalry with defending champion Dayron Robles, as well as taking on in-form Aries Merritt. In last year's world championships, Liu was twice bumped by Robles's trailing arm, which led to the Cuban being disqualified from gold.Liu has been bothered by a back injury in the run-up to the Games, but coach Sun Haiping insists his runner is in good shape. "His back problem is nothing to worry about, but his foot injury which has flared up is worrying me," Sun said. "We are confident despite the injury." Little has been seen of Robles this year with first an injury to his lower back, and then to his lead leg, continually delaying his return to the track. However, his coach Santiago Antunez remains confident he will become the first champion to successfully defend his title since American Roger Kingdom in 1984 and 1988. "He (Robles) is the Olympic champion, and the fastest in the world in the event," said Antunez. "If he loses in London in what is a really tough competition it will be because of a problem that occurs on the day." Bolt, fresh from running the second fastest time in history to retain his 100m title on Sunday, begins his bid to keep his 200m title when the first round takes place. He believes a 200m victory will cement his claim to be regarded as the greatest ever sprinter. "I think I have to wait until the 200 metres and then my fans have got to say 'yes he is' and I have got to say 'yes he is' so I'm never going to say I'm the greatest until after my 200m," said Bolt. Also on the track, Australian world champion Sally Pearson is favourite for the women's 100m hurdles gold.Pearson has emerged as the dominant force in women's short hurdles, racing the fourth fastest time ever (12.28sec) in winning world gold last summer. "If it all goes to plan I should be winning, especially as my personal best is much better than theirs," she said. In track cycling, Britain will be eyeing more gold in the women's omnium through Laura Trott, while Victoria Pendleton could take a second gold of the Games in the women's sprint. The men's football finalists will be decided when Mexico tackle Japan and South Korea meet Brazil.