Trinidad and Tobago tied with Great Britain in first place for the first heat - both registering 3:00.38.
Great Britain's Jack Green produced a storming finish to the third leg to give anchor runner Martyn Rooney a narrow lead.
Rooney held the lead until easing up just before the line, allowing Trinidad and Tobago to take first place, although both teams were given the same time and qualify automatically for the final.
The Bahamas and the USA were also given the same time - 2:58.87 - in the second semi-final, to qualify quickest.
Those four nations will be joined by Cuba and Russia in Friday's final after they also hit qualification times in the heats.
Any faint hopes of Usain Bolt appearing in the 4 x 400m Relay final were ended when Jamaican team-mate Jermaine Gonzales pulled up injured on the third leg.
However, there will be a chance for the South Africa team featuring Oscar Pistorius, after they were given a place in the final on appeal.
Pistorius was due to run the third leg in this morning's heats but did not even get his hands on the baton after team-mate Ofentse Mogawane collided with Kenya's Vincent Mumu Kiilu around the top bend on the second leg.
Both athletes were sent sprawling to the track and although Kiilu was able to resume running, Mogawane suffered a suspected dislocated shoulder and could not carry on.
The Kenyan team were disqualified and South Africa lodged a successful appeal, which saw them added to tomorrow's final in lane nine. LJ van Zyl will replace Mogawane in the quartet.
The decision has delighted Pistorius, who admits he is living a dream by competing on the Olympic Games stage.
He said: 'It's been absolutely phenomenal, just stepping out there again today on the track in front of a crowd like this has been awesome.
'This whole experience has just been mind-blowing for me, I can't describe it in so many words, I'll be speaking for days.'
Elsewhere, it took almost two hours to determine the 12 qualifiers for Saturday's final of the women's High Jump.
With 14 athletes still in contention when the bar was raised to the automatic qualification standard of 1.96m, discussions started between athletes and officials to determine if the additional height would be required to make the cut.
After careful study of the standings, all seven athletes without a failure decided to pass as they were guaranteed to be in the top 12, with three athletes with only one failure doing the same.
That left four women battling for two places and former world junior champion Svetlana Radzivil of Uzbekistan cleared 1.96m at the first attempt to book her place.
Germany's Ariane Friedrich and Greece's Adonia Steryiou had three failures at 1.96m, leaving Russia's Irina Gordeeva to advance courtesy of having just one failure against Friedrich's two at 1.93m.