Jewell Loyd added 28 points and Sue Bird had a WNBA playoff-record 16 assists. Bird had 10 assists in the first half to also break that playoff record. Game 2 of the best-of-five series is Sunday.
Stewart added 15 rebounds and four blocks. She was a point short of the WNBA Finals record set by Angel McCoughtry.
With Seattle holding a two-point lead heading into the fourth quarter after a huge Las Vegas rally, Stewart took over.
She scored the first 11 points of the period to restore a double-digit advantage The 2018 MVP, who missed last season with a torn Achilles' tendon, hit three 3-pointers in the game-changing burst.
McCoughtry scored 20 points to lead Las Vegas, which was playing in the WNBA Finals for the first time since the franchise moved to the Nevada city three years ago. The team was in the finals in 2008 when it was located in San Antonio. A'ja Wilson, the league's MVP, added 19 points.
Seattle is trying to become the third team to win four titles, joining Houston and Minnesota. The Storm were champions in 2004, 2010 and 2018.
The Storm trailed 34-31 with 6:26 left in the second quarter before going on a 26-6 run to close the half. Stewart and Loyd combined for 17 points during that spurt as Seattle scored the first 14 points in the burst.
Seattle increased the lead to 19 to start the third quarter, but the Aces rallied with a huge run of their own to tie it at 67 just before the end of the quarter.
HELPING HAND: Bird broke the playoff assists record 14 she held with Courtney Vandersloot of Chicago. Eight players had reached 11 assists in a finals game before Bird's record performance.
BEST SELLER: Bird had the top-selling jersey in the WNBA this season, the league announced earlier Friday. It's the first time in her career that the Storm guard led the list. Wilson was sixth on the list and Stewart seventh. The Storm were the overall leader as far as popularity in merchandise. The Aces were fourth on the list
MISSING: Both teams will be without key reserves in the finals. Dearica Hamby, the WNBA Sixth Woman of the Year, injured her knee in the semifinals against Connecticut and is not expected to play in the championship series. Seattle sharpshooter Sami Whitcomb left the bubble to be with her wife as she gives birth to the couple’s first child.