Griner had 18 points and 10 rebounds as the Mercury beat the Seattle Storm 78-65 in the season final Sunday night, setting a new WNBA mark for victories.
Candice Dupree added 16 points to help Phoenix (29-5) top the previous record shared by Los Angeles (28-4 in both 2000 and 2001) and Seattle (28-6 in 2010). Penny Taylor and Erin Phillips had 11 points apiece.
"Throughout the year, I could see it," Griner said. "We just kept playing hard. We hate losing, and we just kept going every day. ... We grew and got better as the season progressed, so I definitely saw more wins coming our way."
First-year Mercury coach Sandy Brondello said switching the team's mindset from offense to defense was instrumental in setting the record one she said she didn't necessarily expect.
"I never look at wins and losses when I take over a team like that. It was more about getting the team together where I thought we needed to be (in order) to be successful," Brondello said. "It was team chemistry. And it was buying into playing defense first and playing out of there on the offensive end, which is something new for this franchise.
"They bought in and really embraced what we wanted to do."
The Mercury, who have home-court advantage throughout the playoffs, will host Los Angeles in Game 1 of the Western Conference semifinals on Friday. Phoenix won all five regular-season games against the Sparks, and also beat them in three games in last year's West semifinals.
"They're a tough team you look at their roster," said Mercury star Diana Taurasi, who was limited to just two points on Sunday. "But it's whoever is best at the end of the day, and we're going to have to be that."
Crystal Langhorne scored 16 points to lead Seattle (12-22), which will miss the playoffs for the first time since 2003, and the first time during coach Brian Agler's seven-year tenure with the Storm.
"It's sort of uncharted territory for us," Agler said. "We can debate about why it happened. But just being (in this situation) was not easy. That being said, we talked about it in the locker room. We don't like being here, so we have to come back hungrier next year, so at this time a year from now, we're finding a way to get ourselves in the playoffs."
Phoenix didn't give up the lead after scoring the first six points of the game. They were up 56-49 entering the fourth quarter, and extended the margin to 12 with 8:40 left. Seattle didn't closer than nine after that.
The Mercury shot 54 percent from the field (30 of 66), the 18th time in 34 games that they topped 50 percent. Their season percentage of 48.4 was a league record, beating the old record of 47.4 set by Minnesota last year. The Storm hit just 37 percent (25 of 68) and finished with fewer than 70 points for the 18th time.