Jefferson had a career-high 39 points and 15 rebounds, and the Timberwolves won two in a row for the first time this season, 117-107 over the Western Conference leaders on Wednesday night.
"I think they have kind of turned the corner," Suns coach Mike D'Antoni said.
Amare Stoudemire led Phoenix with 33 points, but only 19 of them came in the last 43 minutes. The Suns, whose 30-13 record is behind only Boston for league supremacy, had their four-game winning streak stopped.
"It's time now. It's time to start turning this thing around and start building for next year," said Jefferson, who helped Minnesota (7-34) take a lead as big as 21 points in the first half.
Leandro Barbosa's 3-pointer near the 4-minute mark cut the lead to 102-92, but Jefferson followed by attacking Stoudemire and drew a foul on Raja Bell for a three-point play to keep the Suns at a distance.
Marko Jaric, Ryan Gomes, Sebastian Telfair, Corey Brewer, Craig Smith and Antoine Walker all had a hand in this, too. The Wolves handled the ball well (11 turnovers) and maintained proper spacing to give Jefferson plenty of options when he drew heavy coverage in the post.
"And sometimes he beat the double team," Gomes said.
Stoudemire started the game with 12 points, including two dunks, and two steals over the first five minutes to lift Phoenix to a predictable 16-6 lead.
The Suns did little for most of the rest of the half, perhaps tired from playing on consecutive nights or simply surprised by such energetic resistance from a team with the league's worst record. They beat Minnesota at home last week by 20 points.
"They started to hit shots, and we stopped defending," Stoudemire said. "Al hit a couple tough shots. Our intensity didn't quite match up to theirs. They turned it up, and we kind of turned it down."
The Suns were beaten here 100-93 on Dec. 8, when Jefferson had 32 points, his previous personal best, and 20 rebounds. The centerpiece of the Kevin Garnett trade, Jefferson has so far been the only unquestionable keeper of all the young guys Minnesota is building around.
He went at Stoudemire aggressively and was all over the boards, grabbing six offensive rebounds before halftime. The Timberwolves held a 20-4 edge in first-half second-chance points.
At one point, Jefferson missed an easy lay-in, but leaped high to tip it in for a 62-42 lead. He grinned on his way back to the other end, obviously realizing it was his night.
Even the struggling rookie Brewer (10 points in the first half) and the sporadically used Walker (seven points, two rebounds, one steal and one assist in 13 first-half minutes) were factors. Jaric, the up-and-down guard, chipped in 15 points in 45 minutes.
The Wolves reached the halfway point of the season on pace for 14 victories, one short of the franchise low. But they came within three in a defeat by Denver on Saturday, and they left Golden State on Monday with a one-point win.
Might this week be a harbinger for a stronger second half?
"I really believe our team's beginning to grow a little bit," coach Randy Wittman said. He added: "I see guys now that don't have the look of who's going to bail us out or who's going to take the big shot?"
This was just one out of 82 for the Suns, a certainty for the playoffs and a legitimate challenger for the NBA title. With their defensive problems and recent history of flaming out before June, however, these two losses to Minnesota might be worth worrying about.
"It's such a tight race in the West to where it may bite us," Stoudemire said. "We've got to start now and just create some type of lead. That way it won't hurt us. Now, it definitely hurts to lose twice against the Timberwolves. We're not taking nothing from them. They played well both times. You got to do a better job of closing guys out."
D'Antoni called it "a crime" that his team shot 56 percent from the field and lost.
"We have a habit of turning it off, and they turned it on, and we couldn't get it back," he said.
- The Suns are 1-7 this season on the road when they trail after the first quarter.
- The Wolves are closer to a comeback by point guard Randy Foye, but Wittman wasn't ready to declare a target return date. Foye returned to practice this week, but his time on the court will be rationed for a while.