The Democratic Alliance (DA) and the Economic Freedom Fighters? (EFF) on Wednesday denied reports of a possible coalition between them after the May 7 elections.
DA leader Helen Zille said a coalition with the EFF would be unworkable because of ideological differences.
"I have always made it quite clear that we will only form coalitions where it is possible to form a coherent government and implement our policy commitments to the public," she said in a statement.
"This would be impossible with the EFF."
EFF leader Julius Malema said his party was not in talks with any other political party about coalitions.
"These reports must be dismissed as mere desperate speculations in an attempt to defocus the EFF's objective of taking government, particularly in Gauteng," he said.
"There are no talks of coalition with anyone because the EFF has only one objective in these elections, and that is to win."
Determined to win
The two parties in Gauteng were reportedly considering a coalition in the province.
Both parties are determined to win Gauteng from the ANC on May 7.
DA Gauteng premier candidate Mmusi Maimane and EFF premier candidate Dali Mpofu have been campaigning around the province.
On Sunday, the City Press reported that the EFF and DA had had informal discussions about the possibility of a post-election coalition government in Gauteng should neither party win an outright majority.
Zille told the paper the party would only consider such a coalition under "very clear conditions". The DA has frequently said it is not opposed to forming coalitions.
Zille often refers to these coalitions as being a part of a broader realignment of opposition politics, which she hopes will gradually eat into the ANC's majority.
"The future of South African politics lies in opposition parties coming together on the basis of shared values to stop the ANC's near monopoly on power, which has inevitably resulted in gross power abuse. More than that, the opposition has a duty to provide an alternative model of governance that works. The only way to do this is for us to build a new majority, step-by-step, starting from the foundation of local government," Zille wrote in 2009. – Sapa; additional reporting by Sarah Evans.