At stake for both sides in the decisive Pool B clash is a place in the quarter-finals and the right to tackle South Africa in Marseille.
Jenkins has been impressed by the impact all three Pacific Islands have made on the tournament and knows Wales cannot afford to be as slow off the blocks against a fired-up Fiji side as they have been so far this World Cup.
"I think the Polynesian teams have been great. Both Tonga and Samoa have improved and I think Fiji will be no different," said Jenkins.
"They've got great individuals and they've got this huge pride in playing for their islands.
"They play a brand of rugby which is unique, based on innate flair. There is no structure around their game.
"There were a lot of positives for us to take out of the match last Thursday but we all realise the importance of the next game, which is going to be a tough one.
"We all know that the best performance we put in has got to be the next one. This is the one."
Jenkins will finalise his side to face Fiji on Monday and expects to have captain Gareth Thomas available.
Thomas had originally feared he would be ruled out of the tournament after damaging his ribs in a late tackle from Stirling Mortlock during Wales' defeat to Australia a week ago.
But both Thomas and Sonny Parker, who injured his groin in the same game, have made good progress and Jenkins is confident they will be in contention for the Nantes showdown.
Jenkins said: "Gareth is going really well in terms of his recovery, and Sonny as well.
"It seems that - like it's been for most of the tournament - I'll be able to pick from a full squad. It is a positive position for us to be in.
"I have a good idea about the team for next Saturday. You need to spend as much time as possible during the week with everyone knowing who's playing.
"There may be players we select who won't be 100%."
The only player unlikely to be available is prop Adam Jones, who has been hobbling around on crutches for nearly a week after a blood vessel burst and leaked onto a nerve in his foot.