Britain's miserable world athletics championships took another nosedive on Tuesday when world champion Jessica Ennis struggled further in the heptathlon and now looks certain to lose her title in the evening session.

These gripping Daegu Games have been shaped so far by high drama and disqualifications, but finally athletic performance was the focus -- though for Ennis it was not the performance she wanted.

Three poor javelin throws where she never got beyond 40 metres left the British athlete trailing Tatyana Chernova with only the 800 metres to go this evening.

The Russian hurled the javelin 52.95m on a steamy Korean morning and now leads by 5,887 points to 5,754.

To stand any chance of retaining her title, Ennis must smash her 800 metres personal best and hope Chernova suffers some sort of disaster in the floodlit race later.

Ennis's slump follows the disappointing showing of compatriot and race favourite Mo Farah in the 10,000 metres and the disqualification for false starts of Britain's women's 400 metres Olympic champion Christine Ohuruogu and sprinter Dwain Chambers.

While Ennis will be desperately disappointed, organisers will no doubt feel relieved that rules nor regulations were the focal points on day four. Those issues have dominated the Games up to now.

On successive nights, the world's fastest man Usain Bolt and then 110 metres hurdles world record holder Dayron Robles were disqualified from competition amid dramatic scenes.

Bolt for bursting early from the blocks in the final, not even getting to run the blue riband race. Robles briefly held gold before he was stripped of it for bumping rival Liu Xiang along the running track in a physical tussle on Monday.

Tuesday night sees more giants of track and field take centre stage as competition approaches the midway point.

Pole vault queen Yelena Isinbayeva enters the action looking to recover the title she won in 2005 and 2007 before losing it two years ago.

To do so, however, she will need to overcome what is being dubbed the "curse of the cover".

On all three days of competition so far, the athletes who featured prominently as cover stars on the official programme suffered ignominious exits that day.

On day one Australian Olympic and defending world pole vault champion Steve Hooker featured, and swiftly exited without even completing one successful jump.

Day two's cover showcased Bolt and Monday was the turn of Robles.

Russia's Isinbayeva should at least get in the air, and is favourite to win back the title. America's Jenn Suhr could run her close, although she is struggling with a back problem.

Anna Rogowska, the defending champion, is near her best with a 4.85m to her credit.

A big performance is expected from world record holder David Rudisha in the 800 metres -- he hasn't lost a race since failing to reach the semi-finals in Berlin two years ago.

The 22-year-old Masai is expected to lead from the front. Sudan's Abubaker Kaki has raced Rudisha close on several occasions and is expected to be the main threat this time too.

Tuesday's action also features the men's 400 metres where LaShawn Merritt, back from a 21-month doping ban, posted the fastest time of the year in heats.