The victory in the quarterfinals of the Women’s Africa Cup of Nations tournament in Morocco means South Africa are guaranteed a top-four finish at the continental championship, which serves as a qualifying competition for next year’s World Cup in Australia and New Zealand.
South Africa now joins Nigeria, Zambia and Morocco in the semifinal and automatic qualification for the World Cup.
There are still two spots for other losing quarterfinal teams, with a continental playoff.
South Africa made their maiden World Cup final appearance in France in 2019 and were heavily fancied to advance again, especially after winning all three of their opening group game at Wafcon, including beating defending champions Nigeria in Rabat last week.
Tunisia were underdogs, lacking experience at this level with this being only their second-ever appearance at the Cup of Nations.
They did prove a gritty opponent and handed South Africa some scary moments, but the result should have been more emphatic for wasteful Banyana.
The outcome was effectively settled when Seoposenwe ran onto a defence-splitting ball and kept her nerve as she calmly slotted the ball past the Tunisian goalkeeper.
Seoposenwe was a constant menace, but her attacking partner Nxolo Cesane got caught offside when she was presented with a good opportunity and when she did find herself in with a chance blasted wide over the top with a yawning goal at her mercy.
South Africa next meet Zambia on Monday in their Wafcon semifinal, followed by Nigeria against hosts Morocco.
Failure to add a second goal meant some nervy moments for South Africa with Tunisia denied a late penalty claim in stoppage time at the end of the game.