The team has endured a woeful campaign so far, having suffered 26-3 and 55-3 defeats to Australia and France respectively.
“We are very disappointed about our performance against France,” said Sephaka on Wednesday.
“We let ourselves and the country down, but I am challenging the players to lift their performance significantly against Wales.”
Sephaka said he expects his team to learn from the mistakes they made in the last two games.
“We are a much better team than the performance we delivered against France, and it is their responsibility to prove that.
“We cannot ignore the fact that France are the Six Nations champions. But we will not hide from the fact that our performance was substandard.”
The team was undone by a series of errors at crucial times, against the French on Tuesday.
Sephaka still hopes the team would improve their 10th place finish in the 2010 World Cup in England.
The team’s appeal against the 16-week suspension handed to captain Mandisa Williams is expected to be heard on Friday.
The loose forward was banned for allegedly striking and making contact with Australian lock Alisha Hewett, in their opening clash last week.
He said they would have a series of video and planning sessions ahead of their final match.
“The women’s season is very short in South Africa and the players are not exposed to such short turnaround times in the provincial season, so we have to give them time off the field to recover.”