Melbourne Victory face rivals Adelaide United in the A-League on Saturday, while in-form Melbourne Heart host Brisbane.
Kevin Muscat's Victory side, who have qualified for the group stages of the AFC Champions League, could take Adelaide's place in third with a win in round 20.
The middle part of the table is heavily congested with Adelaide, Victory are Central Coast locked on 27 points.
Sydney (25) make up the top six, with Wellington and Newcastle (both 23) and Perth (21) not far behind, while even Heart (18) can still make it.
It makes Victory's meeting with Adelaide at AAMI Park an important encounter as they look to cement their place in the top six.
Victory attacker James Troisi (nine league goals) appears to be recapturing the early-season form which had many predicting he would be in Australia's squad for the FIFA World Cup.
With Gui Finkler, Archie Thompson and Tom Rogic, Victory should at least regularly threaten Josep Gombau's side.
Adelaide have won consecutive league games without conceding a goal, but are prone to having their moments in defence.
Bottom side Melbourne Heart have surprisingly developed into the league's form team and take on leaders Brisbane.
David Williams (nine league goals) inspired John van 't Schip's men to a 5-0 thrashing of Wellington last time out.
But Mike Mulvey's Roar side will be desperate to get back to winning ways after once again being beaten by Newcastle last time out.
Sydney could give up their top-six place if they are beaten at Newcastle on Saturday.
Both sides have been inconsistent throughout the season and who turns up at Hunter Stadium could decide who claims a vital three points.
Central Coast are hanging onto their place in the six, but need to avoid a fourth straight loss when Wellington visit.
Ernie Merrick's Phoenix side never got going against the Heart and need to bounce back at the Mariners.
Second-placed Western Sydney will fancy their chances when they travel to Perth on Saturday.