Democratic Alliance leader Helen Zille said a coalition with the Economic Freedom Fighters 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."
The two parties in Gauteng were reportedly considering a coalition in the province.
Both parties are determined to win Gauteng from the African National Congress on 7 May.
DA Gauteng premier candidate Mmusi Maimane and EFF premier candidate Dali Mpofu have been campaigning around the province.
- Watch DA leader Helen Zille speak about why it's better to be a strong opposition than be in a fragile coalition government.
- See News24's Elections Map to track the DA's performance in previous elections.