The Sun, who swept the Sparks in the semifinals last year, take on top-seeded Las Vegas in a best-of-5 semis beginning Sunday.
Alyssa Thomas added 19 points to lead five Sun players in double figures.
"We're really excited, 7-3 in the last two years in the playoffs," Connecticut coach Curt Miller said. "Pretty good 10 game stretch."
The Sun jumped out to a 22-8 lead and led from there after the Sparks shot just 21% in the opening quarter. The Sparks rallied within six before Connecticut finished the second quarter on a 10-0 run. It would have been 12 straight points except Bonner's layup settled on the back of rim and remained there as the teams headed off the floor.
Briann January hit a 3-pointer early in the fourth quarter to give the Sun their largest lead of 20.
Candace Parker had 22 points and 14 rebounds for the Sparks, who were without third-leading scorer Nneka Ogwumike, sidelined with a migraine.
"Nneka does a lot for us as a team on both ends of the court," Los Angeles coach Derek Fisher said. "(We missed) her efficiency offensively on a night when it was difficult to find some rhythm on that end of the floor. She's a really good defender and rebounder, Most importantly her presence from a leadership stand point helps our team to stay centered some times."
Star guard Chelsea Gray was held to just four points and didn't have an assist for the first time since their loss to Minnesota in the last game of the WNBA Finals in 2016.
The Sparks came into the game as the league's top 3-point shooting team at just under 40% but made only 2 of 18 against the Sun, missing their first 12. The 59 points were the fewest this season for Los Angeles.
The Sun, swept by the Sparks during the regular season, scrambled to reach the playoffs after losing their first five games. They beat Chicago by 14 in their first-round game.
Bonner fired up after and-1 layup
Alyssa Thomas threads the needle with a pinpoint bounce pass to DeWanna Bonner, who finishes with an and-1 layup and lets out a roar of excitement.