The Jazz guard scored 32 points -- 13 of them in the fourth quarter and seven during a 10-0 run when Utah turned back Boston's last chance -- to win 110-92 on Friday night and stop the Celtics' 10-game winning streak.
"I tried to take over a little bit," said Williams, who was icing his head afterward. "It was time for me to assert myself a little more."
Carlos Boozer had 17 points and 12 rebounds for his 44th double-double of the season, and Mehmet Okur had 12 points and nine rebounds for Utah. Williams went 17-of-18 from the line and added eight assists and five rebounds as the Jazz handed Boston its worst loss of the season.
"He just kept driving and driving and driving and kept getting fouled and fouled and fouled. He played terrific," Boozer said. "These are the games you circle on your calendar. It was a big win for us on the road."
Kevin Garnett had 15 points, four blocks and six assists for the Celtics, who had a chance to clinch the Atlantic Division by themselves. Instead, they needed another loss by Toronto to earn a top seed in the NBA playoffs.
"It was one of those games, man," Paul Pierce said. "You kind of feel like a little kid out there in the park and the bully came and snatched the ball and there's nothing you can do about it."
Boston played the last three quarters without Ray Allen, who slipped and fell, bruising his left heel at the end of the first quarter; he did not return. Coach Doc Rivers said Allen would make the trip for Saturday night's game against Milwaukee, but "he's going to have to be extremely healthy for me to play him."
Boston cut a 13-point, third-quarter deficit to three points in the fourth, and trailed just 93-89 with 5:25 left. But Williams led the Jazz on a 10-0 run while the Celtics missed a pair of 3-pointers, committed a pair of charging fouls and threw in a flagrant foul by Kendrick Perkins.
"I could tell right off the bat, energy-wise, we weren't our normal selves," Garnett said. "Against a good team like Utah, you've got to get everything oiled up and ready to go from the jump."
The Celtics went four games without trailing in a game before Seattle scored the first basket on Wednesday night. But the Jazz opened a 12-point lead in the first quarter on Friday night and made it 14 in the second.
Utah held the lead even as Boston cut it to five points, 79-74, at the end of three.
The Celtics wore green uniforms with gold trim instead of their traditional home whites on Friday because they're on the road Monday for St. Patrick's Day. ... After the game, there was a concert by Grammy-nominated singer Natasha Bedingfield. ... Jazz coach Jerry Sloan was called for a technical foul for arguing with the referee with 4:18 left in the third. ... Tony Allen had four breakaways in the first few minutes of the second half; twice he had to pass the ball back out to avoid a blocked shot, once he converted a layup and on the last he made an acrobatic dunk. ... Boston's previous biggest loss was 95-83 on Jan. 9. ... Although the Jazz are 28-3 at home -- winning 18 in a row -- they are just 16-20 on the road.