Melbourne United have likely been plunged into a sudden-death duel with the dangerous Tasmania JackJumpers after the free-swinging Illawarra put a dent in their NBL title ambitions.

To finish fourth United almost certainly needed to win Sunday's home clash with the defending champions, who are already out of post-season contention ahead of this week's final round.

And they led by nine to start promisingly before things deteriorated, the Hawks bursting clear and then defying a brief final quarter rally to outgun United 100-91.

United (19-13), again without head coach Dean Vickerman (appendicitis) and bench spark Shea Ili (leg), began the season with nine straight wins.

But again on Sunday they battled for any flow on offence and came unstuck at the other end, with on-ball defence an issue that led to some easy Hawks (12-20, eighth) buckets.

The result all but locks United into fifth, with Perth one win clear in fourth and with an extra game, plus percentage, in hand.

It sets up a mouth-watering post-season knockout clash with the JackJumpers that will determine who plays the loser of a third-against-fourth playoff to decide the semi-final match-ups.

"Tonight, it was clunky," United assistant coach Rhys Carter said of his side's offence.

"A few times we just weren't on the same page.

"I felt like we let it get to us. A few bad possessions, you could feel the vibes drop.

"We have to find a way to get out of those small holes and not make them big holes.

"It wasn't our best tonight but we still think our best is good enough."

JaVale McGee (22 points, 13 rebounds, four assists, two blocks) was commanding for the Hawks while the evergreen Todd Blanchfield (11 points, three-of-four triples) was effective and Tyler Harvey (18 points, five rebounds) was again in the groove.

Tyson Walker (30 points, four assists, two steals) tried to spark his team and his triple capped a five-point run to begin the final quarter that brought the margin to nine.

But Will Hickey (13 points, three steals) bobbed up with a bucket, steal and assist for Blanchfield on successive plays, before a Wani Swaka Lo Buluk (11 points) triple boosted the visitors to safety again.

"Exciting to see them go and enjoy themselves, playing the game they love, playing for each other and making winning plays ... and the ball went in for us," Hawks coach Justin Tatum said.

"Our record doesn't show who we are in the locker room and what we do every day."