Pisi limped off nursing a hamstring injury during Samoa's 49-12 Pool D victory over Namibia on Wednesday.
However, he has been named to start in a side showing just one change from that game, with flanker Ofisa Trevaranius replacing Taiasina Tuifua.
"Today he is 80%, and he might be 99% by tomorrow," Samoa coach Titimaea Tafua said. "And we still then have a whole other day before the match."
Tuifua, meanwhile, features on the bench despite suffering a rib injury just minutes after Pisi was hurt against Namibia.
Victory for Samoa over Wales - a feat they accomplished at the 1991 and 1999 World Cups - would strengthen their quarter-final claims from a punishing group that also features South Africa and Fiji.
But the game comes just four days after the South Sea Islanders faced Namibia, whereas Wales have had a week to recover following their 17-16 loss to South Africa in Wellington.
"It's pretty harsh," Samoa skipper Mahonri Schwalger said. "But it is the nature of the tournament. It is about which team can recover quickest and front-up."
Samoa displayed much of their trademark physical power in beating Namibia, a performance that included three tries for Leicester wing Alesana Tuilagi.
Wales are braced for a similar approach at Waikato Stadium, where Samoa are guaranteed huge support.
Samoa lock Kane Thompson said: "We are excited. It's a huge game for us, but it's like every game in this World Cup. Every game is a quarter-final for us.
"We know if we're going to progress in this tournament we have to win pretty much every game.
"We will play our usual style. Most teams around the world know how we play now. We're pretty physical.
"And we can't let the four-day turnaround affect us. If we let things like that affect us, we won't be focusing on the task at hand. It definitely won't be an excuse."
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