WHEN Sonia O'Sullivan presents herself on the starting line for this morning's 1,500 metres heats (3.10 pm, Irish time), she will be facing the biggest challenge of her career to date.

For much of the last four years she has had a reputation as one of the most self assured athletes on the circuit, a skilled professional who churned out top-class performances with the regularity of a conveyor belt.

Now the scene is vastly different. Three days ago she sank to the lowest level of her career when dropping out of the 5,000 metres final.

The depth of the disillusionment which drove her into a tunnel, when pride and pedigree demanded that she see out her ordeal to the bitter end, was such that it's going to take a massive demonstration of character for her to deliver on the rating as favourite for the 1,500 metres title.

In the search for sustenance, supporters point to the evidence of the World championships in Stuttgart three years ago when, after coming undone in the 3,000 metres final, she bounced back to take a silver medal at the shorter distance.

The memories of that brave recovery are encouraging, but the parallels are a little blurred. In Germany she found herself at the sharp end of a Chinese coup but still salvaged some pride in finishing fourth.

No such comfort could be derived from last Sunday's run. There was little evidence of either the pace or the power which has propelled her to victory on so many occasions and established her as Europe's top female athlete for the last two years.

Later, it was stated that the reasons for that dismal run were physical and the images portrayed a picture of a person in some discomfort, even at a relatively early stage of the race.

Since O'Sullivan is not staying in the Olympic village - she is living in a privately-rented house somewhere in the city - it has not been possible to assess the rate of her recovery from the stomach problems which were advanced as the cause of her collapse in Sunday's race.

But, on the evidence of her appearance at Monday's press conference, she is still dangerously fragile. Frail and, at times, filtering, she did not convey the impression of a person who had recovered from the trauma of the previous evening.

Whether the intervening period of 48 hours has been sufficient to restore her to full health remains to be seen. But in a situation in which normally she would require some time to recover from that ordeal, the risks are now obvious.

The first priority is to run sufficiently well to qualify today and hopefully she will do it with style to dissipate the self doubt which was all too obvious last Sunday evening.

On the face of it, the task ahead should not be too difficult. The top six in each of the three heats, plus the six fastest losers, go through to the semi-finals tomorrow evening and, barring another calamity, it should prove relatively straightforward.

Drawn in the second of the three heats, O'Sullivan's principal rivals will be Carla Sacramento of Portugal and the Russian Lyudmila Borisova, whose seasonal best of four minutes 2.13 seconds is three seconds slower than that of the Irish woman.

Also included are Vicki Huber (US), a former teammate of O'Sullivan at Villanova University, Naomi Mugo (Kenya) and Cathilina Gheorghi (Romania). But the depth in quality in the field is scarcely enough to cause further trouble for O'Sullivan.

Sinead Delahunty's task in the first heat is significantly more difficult. Among those lining up with her will be the reigning champion, Hassiba Boulmerka of Algeria and the Russian Svertlana Masterkova, a surprising winner of the 800 metres title on Tuesday.

On that occasion Masterkova upstaged Maria Mutola and Ana Qoirot and, from a situation in which she scarcely rated in the pre-Atlanta calculations, she has now emerged as a real threat for the big double.

Cormac Finnerty, Ireland's only representative in the 5,000 metres, has been drawn with a batch of African runners in the second heat while in the 200 metres preliminaries Gary Ryan is in heat four.

Today's programme will also mark the start of the second phase of Michael Johnson's attempt to create Olympic history and become the first to win the 200 and 400 metres titles.

It's known here as the Johnson and Johnson act and the manner of his immaculate performance in the 400 metres final on Tuesday was convincing enough to suggest that it can be done.

For a man who enjoys such enormous popularity in Europe, Johnson's rating in the pecking order of American sporting stars is remarkably low. Now he senses that it could be about to change.

"There was one occasion after I competed in Barcelona four years ago when I found myself surrounded by autograph hunters," he said. "And I overheard an American at the back of the crowd ask `who is that?'."

"That was hurtful, but I think things are now beginning to change. I know I had the support and the goodwill of the American people on Tuesday. And I believe that it's going to continue for the rest of the week."

"The 400 metres is over. Now it's a case of focusing on the 200 - and I'm looking forward to it. In this event there is no holding back. You go from the gun and it's exciting."

Barring the way to the double, which he openly covets, is Frank Fredericks the Namibian who ended Johnson's two-year unbeaten sequence at the distance in Oslo last month. That was a shock to the great man's pride, but he is adamant that revenge is now imminent.

"I made mistakes in Oslo and it cost me the race. This time there will be no errors."