The defending champions are currently three points off top spot at the halfway stage of the season, but their hopes were hit by a 2-0 defeat to Benfica last weekend.
That result gave their rivals a clear advantage at the top of the table, while Sporting are sandwiched between the leaders and Fonseca's side.
The head coach was far from happy with the outcome at the Estadio da Luz, with four Porto players booked and defender Danilo shown a red for two yellow-card offences - compared to only two cautions for Benfica players.
And in an extraordinary outburst at his media conference on Friday, Fonseca claimed that there is a conspiracy against his side.
"The treatment given to FC Porto is not the same as the other clubs," he said ahead of this weekend's clash with Vitoria Setubal.
"It is notorious, and only those who do not want to see do not see.
"There is a concerted effort to weaken FC Porto and promote the other two clubs' (Benfica and Sporting's) campaign."
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