Morceli, the world record holder and Olympic champion, could not afford to relax as the Spaniards Reves Estevez and the former Olympic champion Fermin Cacho controlled the race from the front until the last few metres when Morceli took the lead.
The Algerian, who was never out of the leading pack, won in 3min 38.82sec ahead of Cacho and Estevez, who both clocked 3:38.86.
Morceli's main rival for the title, Morocco's Hicham El Guerrouj, came through his semi-final a few minutes earlier, but had a comparatively less severe test, winning in 3:38.92 to set up a fascinating duel in tomorrow's final.
El Guerrouj, who has run four of the five fastest times of the year, will be especially keen to win the world crown for the first time after his Olympic hopes ended in last year's final in Atlanta when he fell.
Kenya's Laban Rotich, the second fastest man in the world this year, also looked in good form and qualified comfortably in 3:38.92.
Sonia O'Sullivan aims to complete the first leg of a World Championship double knowing that fortune finally looks to be turning in her favour.
A year ago, O'Sullivan was struck down by a debilitating bug which left her a pale shadow of the runner who had dominated the world all season.
Instead of leaving Atlanta with the anticipated golden haul, she departed the United States in tears, forced to drop out of the 5,000m final and not even getting through the first round of the 1500m.
Those problems seemed to continue into this year as she left Sheffield's Don Valley Stadium in June after finishing so far behind Britain's Kelly Holmes in the 1500m that she was virtually being written off.
But, six weeks later, the 27-year-old from Cobh, whose main aim when she flew out to Athens was defending her Gothenburg 5,000m crown, is now favourite for today's 1500m final.
First, Holmes, the world No 1, crashed out with an Achilles problem, then defending champion Hassiba Boulmerka of Algeria failed to show for her heat and on Sunday the Olympic gold medallist Svetlana Masterkova also succumbed to her injury problems.
O'Sullivan had singled out Holmes and Masterkova as her two main rivals, and after doing the minimum possible so far, she now has a confident aura.
"I think I've proved my fitness now and I've not got down to my last training session yet," said O'Sullivan, who ran her fastest race of the season in the semi-final, despite lying 10th with 400m to go.
Her main threats are likely to be American Regina Jacobs and Russia's Olga Nelyubova. But the acceleration O'Sullivan showed in moving through almost the entire field in the space of 150m before settling for a gentle finish will have been noted by her rivals and if the race is run slowly, that speed could be crucial.
"Hopefully things will work out. I've had a day off today and we'll see how it goes from here," O'Sullivan said. "The good thing so far is that I haven't really been nervous at all and if I can stay relaxed, it will help.
"All I want to do is run my best possible race and take it from there. I've worked hard all year and now I'm here I'm going to make the most of it."