Luka Doncic led a stunning Dallas Mavericks rout of the top-seeded Phoenix Suns while the Boston Celtics eliminated the defending champion Milwaukee Bucks on a day of NBA playoff upsets on Sunday.

In Phoenix, Slovenian superstar Doncic led a total eclipse of the Suns as Dallas clinched a 4-3 series victory with an extraordinary 123-90 thrashing of the Western Conference favourites.

The Suns had entered the playoffs boasting the best record in basketball after a 64-win regular season and were tipped by many to return to the NBA Finals for a second straight season.

But the Suns' dreams of a championship were incinerated by a red-hot shooting performance from Doncic and the Mavs, who will now advance to a Western Conference finals showdown with the Golden State Warriors starting on Wednesday.

Doncic finished with 35 points, 10 rebounds and four assists while team-mate Spencer Dinwiddie provided scoring support with 30 points, including five three-pointers.

Jalen Brunson also posted double figures while Phoenix were left pondering a no-show by their two stars Devin Booker (11 points) and Chris Paul (10).

The Mavericks led from start to finish and at one stage in the second half had sprinted into a 46-point lead.

Their 57-27 half-time lead was the biggest ever in a decisive game seven in the NBA playoffs.

"Amazing," Doncic said. "I don't know what to say but this was amazing to win.

"Everybody played as hard as they can. We didn't have any days off. An incredible team win. I don't have nothing more to say."

Phoenix coach Monty Williams said his team had saved their worst performance of the season for their most important game.

"I just told them how badly I hurt for them," Williams told reporters shortly after addressing his team.

"I know they didn't want to play that way. We basically played our worst game of the season tonight. I know how bad they wanted it, they just could not execute tonight, could not make a shot early on.

"And that messed with us a little bit, and Dallas played their tails off from start to finish.

"It's a part of manhood. There are days when it doesn't go your way and you've got to stand there and show character and integrity and take it. That's life."

- Williams stuns Bucks -

There was a similar sense of deflation amongst the Bucks players after their reign as NBA champions ended in a convincing 109-81 game seven defeat to the Celtics in Boston.

Celtics forward Grant Williams produced a deadly display of scoring from three-point range at the TD Garden as Boston advanced to a conference finals showdown with the Miami Heat with a 4-3 series win.

Williams's shooting from beyond the arc was emblematic of Boston's superiority from the field as the Bucks missed a slew of wide-open looks.

Bucks star Giannis Antetokounmpo finished with 25 points, 20 rebounds and nine assists for Milwaukee.

But the Celtics' three-point shooting accuracy proved the difference between the two teams as Boston completed a comeback from 3-2 down in the best-of-seven series.

Boston dropped 22 of 55 from downtown, in stark contrast to Milwaukee, who converted just four of 33 attempts from beyond the arc.

Williams led the three-point blitzkrieg with seven threes in his 27-point haul. Jayson Tatum made five-of-nine threes to finish with 23 points.

Payton Pritchard also got in on the act, with four three-pointers from the bench in a tally of 14 points.

Only one Milwaukee player, Bobby Portis, made multiple three-pointers. Portis finished with two in his 10-point haul.

Celtics coach Ime Udoka revealed he had encouraged Williams to shoot more aggressively, suggesting the Bucks had "disrespected" the forward by allowing him so much room to operate.

"I told him let it fly. I said 'They're disrespecting you more tonight than earlier in the series,'" Udoka said.

Williams said he was only too happy to accept the challenge.

"Everyone kept saying 'Let it fly, keep shooting'. So I thought 'All right, they’re encouraging it, might as well take advantage.' It was fun. It was just great that we got a win," Williams said.

Milwaukee coach Mike Budenholzer made no excuses for the defeat, saying Boston had been the better team over the series.

"I couldn't be prouder of our team," Budenholzer said. "We've had a lot of success together and tonight we hit a wall and met our end."

Boston face top seeds Miami in game one of the Eastern Conference finals on Tuesday in a rematch of their 2020 series.

Miami won that series 4-2 to advance to the NBA Finals.