Western United's dream debut A-League season continued as Alessandro Diamanti's stunning free-kick secured a 1-0 win over Brisbane Roar and a semi-final berth.

In the second upset of the post-season, following Perth Glory's elimination match victory over Wellington Phoenix, Diamanti struck on Sunday to set up a last-four meeting with Melbourne City.

United, who finished fifth in the regular season, led after 21 minutes when the former Italy international swept in a sensational 25-yard effort, with Jamie Young only able to help the ball into the top-left corner.

Brisbane could have equalised before half-time but Matthew Ridenton sent his left-footed volley just wide.

Young kept his side in the gane with a brave double-save from Josh Risdon and 37-year-old Diamanti, before the Roar came again.

Jay O'Shea shot wastefully straight at Filip Kurto and then Brad Inman drove forward and blasted against the post, but United held on for a historic win.