Samba gifted Fulham two goals in Monday night's 3-2 defeat at Craven Cottage, a result that has left Rangers seven points adrift of safety with seven matches remaining.
The Congo centre-back was involved in a spat with supporters on Twitter following his performance, but Redknapp insists it was a one-off.
"Samba had a nightmare to be fair, but he was absolutely brilliant when we won at Southampton," Redknapp said. "It was one of the best performances from a defender I've seen for years. He's a top player who could play for any side in the top four.
"He had a bad night against Fulham, but he's not the first defender I've seen make mistakes. On Sunday I'm sure we'll see the Samba we're used to seeing."
QPR's chances of escaping relegation look bleak following successive defeats to Aston Villa and Fulham, but Redknapp refuses to give up hope of masterminding survival.
"When I stop believing, the players will stop believing. If I send out the wrong message they'll down tools, thinking we've had it," he said.
"I don't feel we've had it. I couldn't tell you the pleasure keeping QPR up would give me. It would be fantastic. We have to keep going - we've got to beat Wigan. There's nothing else for it, we have to beat them.
"Nothing else but a win would be any good to us. We need to win and keep winning. We need to win four or five games - it's a tall order but not impossible.
"But if we don't win on Sunday we won't give up because there'll be six games left and we could still win all six. You don't give up, you have to keep going."
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