Brisbane were beaten 2-0 by second-placed Sydney in their final match of the regular season at Moreton Daily Stadium on Saturday.
Goals from Kosta Barbarouses and Harry Van der Saag opened the door for the Mariners to move above the Roar and take third place, an opportunity they grabbed with both hands.
Jack Clisby struck in the first half and Marco Urena sealed the victory in stoppage time at Central Coast Stadium.
Alen Stajcic will pit his wits against fellow former Matildas coach Ante Milicic in an all-New South Wales clash next Saturday.
The Roar will have an extra day to prepare for a home showdown with Adelaide United a week on Sunday, when a place in the semi-finals will be up for grabs.
Newcastle Jets moved off the bottom of the table courtesy of a 1-1 draw with Perth Glory, A Jonathan Aspropotamitis own goal giving them a point after Bruno Fornaroli opened the scoring.
That point moved the Jets above Melbourne Victory, while Perth remain ninth.
Western Sydney Wanderers will host Macarthur FC in the 2020-21 season opener on December 27, it was revealed on Tuesday.
The other opening games will see Western United host Adelaide United, Wellington Phoenix visit Brisbane Roar, Sydney FC take on Melbourne City and the F3 Derby between Central Coast Mariners and Newcastle Jets.
The opening round would seem like just another game, but history shows otherwise.
With the help of Opta data, we assess just what round one means in the A-League.
Opening loss and trophies become difficult
Incredibly, only twice in A-League history has a team that lost in round one gone on to be crowned champions or premiers of that season.
The first instance came in 2011-12, when the Mariners were edged by Brisbane in round one, but went on to win the Premiers' Plate, finishing two points clear of the Roar.
Melbourne Victory were the second, going on to be crowned champions in 2017-18 despite a 1-0 loss to Sydney FC in round one.
In all, six of the nine teams to do the A-League double of being crowned premiers and champions in a single season have done so far after winning in round one. The other three drew. Of the 15 champions, seven have won and seven have drawn, to go with Victory's 2017-18 defeat, in round one. Nine of the 15 premiers have won in the first round, alongside five draws and the Mariners' loss.
Round one looks hugely important in the A-League.
Fast-starting Sydney, Glory woes
Sydney have scored more goals (21) in round one games than any other team in the competition, while earning the most points per game (1.7) of teams to have featured in at least five seasons.
The Sky Blues have lost just two round one matches in their history, and the most recent of those was in 2012-13. They face a huge test to begin 2020-21 season, hosting Melbourne City in a Grand Final rematch at ANZ Stadium.
On the other hand, Perth Glory have lost more games in round one than any other team in A-League history with seven.
In contrast to Sydney, Perth are winless in their past five openers, their last win coming in 2014-15.
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.
The 2019-20 A-League campaign has been suspended since March due to the COVID-19 crisis, but Australia's premier football competition is set to restart on July 16 followed by a five-game finals series – the Grand Final to take place on August 23.
A derby between Melbourne Victory and Western United – originally scheduled for AAMI Park in Melbourne – will be played at a stadium still to be determined due to a spike in coronavirus cases in the state.
Six venues will be used in New South Wales and Queensland with the potential for more to be added subject to government regulations – ANZ Stadium (NSW), Bankwest Stadium (NSW), Central Coast Stadium (NSW), McDonald Jones Stadium (NSW), Jubilee Stadium (NSW) and Cbus Stadium (QLD).
"Over the past three months all stakeholders have been working hard together with one goal in mind – To finish what we started," FFA's Head of Leagues Greg O'Rourke said on Wednesday.
"We know the COVID-19 environment has been very difficult for many people in our community and we hope that delivering an extravaganza of matches will allow our fans to basically see a game everyday will be a unique experience for those who have stood by us.
"As we are a truly national game, the current border controls means that some of the match day venues may need to change, however we have secured venues in both a NSW Hub model and a hybrid hub model which has most games in NSW but allows for a few games in other states.
"Whilst we don't see the draw changing, the planning we have undertaken provides us with agility to shift between certain states if required. This version of the draw displays our current venue plan.
"We will continue to work with the Federal and State governments to ensure the health and safety of our players, coaches and other stakeholders and have detailed and practical protocols in place for all parties involved in training and match days.
"I am excited to see us kick off the restart and ultimately finish off what we started last October, with our priority always being to be in a position to crown our Hyundai A-League premiers and champions for this season."
Defending champions Sydney FC top the table by eight points through 20 matches, though second-placed Melbourne City have played three games less.
Premier Annastacia Palaszczuk announced on Wednesday that up to 2,000 fans can return to stadiums beginning on the weekend.
The Gold Coast Titans are hosting the St George Illawarra Dragons at Suncorp Stadium in Brisbane on Saturday.
Brisbane Roar could also welcome fans to the ground when the A-League season resumes in mid-July, while it is also good news for the AFL.
"Up to 2,000 fans can return to Queensland's stadiums from this weekend," a tweet from Palaszczuk read.
"Queenslanders have done an incredible job smashing the curve, and codes have done their part to develop COVID Safe plans to keep fans safe.
"It's another positive step on our road back to normal and I know Queenslanders are eager to get back to games to cheer on our teams."
Australia has more than 7,300 cases of coronavirus, including 102 deaths.
Titans chief executive Steve Mitchell welcomed the move, while praising Queensland's handling of the COVID-19 crisis.
"This is obviously great news for the Titans and the NRL, but for fans of sports across Queensland," he said.
"The past few months have been tough on everyone, and the hard work done by the NRL in getting our competition restarted gave us a feeling that life was finally returning to normal.
"Thanks to the premier and her government in containing the virus in Queensland, we are now at the stage where we are able to take the next step on the journey back to normal life, with 2,000 spectators able to come to Suncorp Stadium on Saturday to enjoy a day at the footy.
"This is a great reward for Titans fans who have been unable to get along to support their team at games, and obviously a big boost for our playing group as well to be able to run onto the field and see and hear the support they have from the Titans Legion.
"This is a wonderful leap forward, but we are not yet at the ultimate goal of a return to full crowds at sporting events. We will continue to be guided by the government and the NRL as we all work towards that goal. There will still be a number of public health restrictions and protocols in place at the venue to ensure the health and safety of our fans.
"But this is certainly a positive milestone for us, the NRL and sports fans in Queensland, and the Titans are delighted to be the first Queensland team to be back playing in front of our supporters this weekend."
The Premier League clubs and A-League outfit Brisbane Roar were due to play in Queensland in July ahead of the 2020-21 season.
However, those fixtures were unsurprisingly postponed on Wednesday due to COVID-19, which has killed more than 177,000 people worldwide.
"It is a real shame that we will no longer be able to make it over to Queensland this summer, however, it is the right decision to postpone given the circumstances," West Ham manager David Moyes said in a statement.
"The players and I were really looking forward to seeing our fans and the people of Australia, especially as there was great excitement and support for the tournament.
"This is a situation that has affected the entire world, though, and the only thing that matters at this time is that people stay safe and well.
"We hope the tournament can be rearranged at a later date once things are back to normal, and we look forward to visiting Australia in the future."
Australia has seen more than 6,600 coronavirus cases, including 74 deaths.
Fowler and his assistant Tony Grant will not return to the club when the season resumes, it was confirmed on Monday.
The former Liverpool forward oversaw 11 wins in 24 games at the helm of Brisbane.
"On behalf of myself and Tony, I would like to thank the club for the opportunity, and the Brisbane Roar FC fans for the warm welcome and support for me and the team," Fowler said.
"We are very proud of what we achieved."
Brisbane were fourth in the table when the season was suspended in March due to COVID-19, with the campaign set to restart next month.
"Robbie has not only provided stability but a refreshing injection of knowledge and insight into the Roar," Roar deputy chairman Chris Fong said.
"We are disappointed that the global pandemic has impacted our plans together but completely understand that family comes first during these difficult times."
