Showman Bolt needs to guard against complacency

Usain Bolt will need to guard against complacency as he begins his bid to win the 100 metres crown while double Olympic champion Mo Farah goes for gold in the 10,000 on the first day of the World Championships on Saturday.

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Track and field, beset again by a spate of positive doping tests to some of the sport's biggest names in the build up to the global showpiece, needs a big performance from its greatest showman.

Jamaican Bolt infamously false-started and was disqualified from the 2011 world final two years ago in Daegu, South Korea, a race won by compatriot and training partner Yohan Blake, who is injured and not in Moscow to defend his title.

World record holder Bolt's sprint dominance was underlined at last year's Olympics in London when he repeated his Beijing feats of 2008 by winning triple gold, including the 4x100 relay.

Bolt will line up in the evening's first round heats with the semi-finals and final on Sunday.

With Blake absent, along with American Tyson Gay, the fastest man in the world this year who has tested positive for a banned substance, Bolt's chief rival will be double 2005 world champion Justin Gatlin.

Farah, like Bolt, is a hot favourite to complete a world track double and repeat his Olympic feats when he stormed to 5,000 and 10,000 gold.

The Briton will also be wary of a joker in the pack after Ethiopian Ibrahim Jeilan beat him into silver over 25 laps in Daegu, before Farah claimed his first world title in the 5,000 later in the championships.

The first gold medal of the championships will be decided in the women's marathon with holder Edna Kiplagat of Kenya facing a stern test of her credentials from Ethiopia's Olympic champion Tiki Gelana.

Olympic decathlon champion and world record holder Ashton Eaton begins his quest for a first world gold after losing out to compatriot Trey Hardee two years ago.

The younger Eaton turned the tables on Hardee at the London Games last year, winning by a 198-point margin.

The first day of decathlon features the 100 metres, long jump, high jump, shot put and 400.