Dirrell was beaten by Carl Froch last October
Andre Dirrell has become the third boxer to withdraw from the Super Six super-middleweight tournament after suffering neurological problems.
The 27-year-old American was scheduled to fight Andre Ward on 27 November.
Jermain Taylor and Mikkel Kessler had already pulled out, replaced by Allan Green and Glen Johnson respectively.
Arthur Abraham is now almost certain of a place in the semi-finals, regardless of the result of his fight against Britain's Carl Froch next month.
Germany's Abraham beat Taylor and Dirrell in his first two Super Six encounters, while Nottingham's Froch beat Dirrell in his first bout before losing to Kessler last time out.
Abraham v Froch, which is set to take place in Helsinki on 27 November, is for the WBC super-middleweight crown Kessler took from Froch in April before succumbing to an eye injury and vacating the title.
Having won his two Super Six fights against Kessler and Green, Ward is already a confirmed semi-finalist.
The Super Six tournament was launched last year amid great fanfare but the various withdrawals and postponements have served to erode its credibility.
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