Meanwhile, Mercury guard Diana Taurasi is in the lineup for the single-elimination game at Seattle (3 p.m. ET on ABC).
The two U.S. Olympic teammates and former league MVPs have been out since early September, Stewart with a left foot injury and Taurasi with a left ankle injury.
Stewart, who has averaged team highs with 20.3 points and 9.5 rebounds this season, last played Sept. 7. Taurasi, who has averaged 15.2 points and 4.9 assists, last played Sept. 6. Stewart, 27, was the WNBA Finals MVP last year as the Storm won the franchise's fourth league title, and she also earned that honor when they won the championship in 2018.
"We've practiced without [Stewart] and had a game without her the last time we played Phoenix," Seattle coach Noelle Quinn said, referencing the Storm's 94-85 victory over the Mercury on Sept. 17. "We've been preparing as if Diana is going to play, but knowing how the team plays without her is beneficial to us. I think the approach remains the same with our energy and our effort."
Taurasi has been limited to 16 games this season because of injuries and didn't play in any of the three regular-season games between the Storm and Mercury. Seattle won the series 2-1, with guard Jewell Loyd scoring 37 points in the Sept. 17 victory.
As the No. 4 seed, the Storm had a first-round bye. No. 5 seed Phoenix beat No. 8 New York 83-82 on Thursday. The Mercury got a career-high 21 points from guard Sophie Cunningham, including six 3-pointers, which helped offset Taurasi's absence. But Taurasi's experience -- the three-time WNBA champion has played in 61 career playoff games -- is irreplaceable.
"The minute Dee steps on the court, it changes everything," said Taurasi's longtime friend and other U.S. Olympian, Storm point guard Sue Bird, a four-time league champion who has appeared in 53 career playoff games. "So we've had to talk about two different Phoenix teams, and get our minds wrapped around that. But it is good that we played them recently."
Phoenix coach Sandy Brondello repeated Saturday the same thing that she has said for the past week regarding Taurasi's status: The team will give her up until nearly game time to make a decision on whether she will play.
"She'll determine whether she thinks she's able to go or not," Brondello said. "She deserves that, to give her as much time as possible. She knows whether she can help us or not. So that's why I leave it in her hands, along with the medical staff."
Seattle and Phoenix have had some epic playoff battles over the years. And with Bird nearing her 41st birthday in October and Taurasi having turned 39 in June, there's always a sense of wanting to see the two keep matching up for as long as they can. Bird is in her 18th WNBA season and Taurasi her 17th.
This season will end for one of them Sunday.
"I can appreciate a rivalry. I can appreciate epic games throughout the course of players' careers," Bird said. "I appreciate the storylines of it, and I love that that's the conversation. But as a player? No, you just go out there and it's this one game that's unique to this year."