Claiming an historic win over Argentina - one of the favourites for gold - got the Olyroos' return to the Games for the first time since 2008 off to a flying start, while the Spanish side toiled to a goalless draw with Egypt.
Coming into the match as affirmed outsiders, Australia wrote one of the first underdog success stories of the year-delayed 2020 Games with a deserved win over ten-man Argentina at the Sapporo Dome on Thursday.
Goals from Lachlan Wales and Marco Tilio - a teenager who last month starred in Melbourne City's first A-League grand final win - saw the Olyroos rewarded for their focused pressing and energetic approach.
When Argentina's Francisco Ortega lost his composure to pick up two yellow cards in as many minutes at the end of the first half, Graham Arnold's young side seized the initiative, having already earned an early lead.
A talented opposition attack could find no way past the Aussies' organised backline, allowing Australia to add to their solitary Olympic football win so far this century - over Serbia and Montenegro at the 2004 Athens Games.
Also manager of the senior national team, Arnold - himself an Olympian at Seoul '88 - had overseen several friendly defeats in the build-up to these Games, including to the Republic of Ireland's Under-21s and the Under-23 teams of Romania, Mexico and local rivals New Zealand.
Therefore, a morale-boosting victory for his squad comprised of A-League youngsters and Britain-based hopefuls now offers hope that they can emerge from a challenging group that also features Egypt and Sunday's opponents, Spain. With only the top two going through to the quarter-finals, they still need more points to progress and perhaps go on to surpass their best-ever Olympic performance - a fourth-placed finish at Barcelona '92.
It was in those iconic Games, in the Catalan capital, that Spain claimed their one and only football gold medal, but a strong current squad got their campaign for a belated second success off to a slow start in midweek.
A familiar tail for the Spanish side - as a team featuring several players who made it to the Euro 2020 semi-finals earlier this month hogged the ball but produced few clear chances against resolute Egypt - means that the pressure is already on to pick up maximum points, as they will face Argentina in their final Group C game.
Though Dani Ceballos thumped a first-half shot against the post, Luis de la Fuente's men were only able to tally five attempts on target and must improve quickly if they are to return to the podium for the first time since receiving silver as runners-up at Sydney 2000.
Of their six senior stars who featured frequently at the Euros - including Dani Olmo, Mikel Oyarzabal and goalkeeper Unai Simon - the most controversial selection has been Barcelona's teenage tyro Pedri.
While his club coach Ronald Koeman publicly opposed the tireless playmaker's inclusion, after he became the first 18-year-old to start six games in a single Euro or World Cup finals, Pedri remains a shining hope for De la Fuente's typically possession-based side.
Coming into his 67th game in less than a year, much pressure sits on his slim shoulders, as Spain must now overcome an impressive Australian team with nothing to lose - and only the top two teams in each section will progress to the knockout phase.
Ahead of Sunday's second round of Group C fixtures, Spain must deal with a couple of injury concerns that arose from their opening game. Less than 20 minutes into the draw with Egypt, Barcelona defender Oscar Mingueza suffered an injury that saw him withdrawn, after apparently overstretching when clearing the ball.
Mingueza instantly pulled up, before being replaced at right-back by Real Madrid's Jesus Vallejo, who is now favourite to take the 22-year-old's place in the back four.
Captain Dani Ceballos was seen hopping around with his boot removed at the end of the first half, having sustained an ankle injury, so he may be replaced by Carlos Soler or Jon Moncayola if he does not manage to recover in time.
In attack, meanwhile, Mikel Oyarzabal was relatively ineffective, so Wolverhampton Wanderers striker Rafa Mir may be given the nod by Luis de la Fuente after a cameo from the bench last time out, with Marco Asensio and Dani Olmo set to provide support.
By contrast, Australia lack such top-level experience in their squad, but named senior international striker Mitchell Duke as one of their overage players and the 30-year-old forward is poised to start up front again.
Duke is expected to link up with Daniel Arzani of Manchester City and Riley McGree - currently on loan at Birmingham City - while Marco Tilio will hope to be promoted to the starting XI, having scored shortly after his arrival as a substitute in the win over Argentina.
Australia Under-23s possible starting lineup:
Glover; Atkinson, Deng, Souttar, King; Arzani, McGree, Metcalfe, Genreau, Wales; Duke
Spain Under-23s possible starting lineup:
Simon; Vallejo, Torres, Garcia, Miranda; Merino, Pedri, Soler; Olmo, Asensio; Mir
We say: Australia Under-23s 0-1 Spain Under-23s
In a game where they are set to boss both possession and territory throughout, Spain must tackle that familiar problem of failure in the final third.
They should get off the mark for the tournament this time, but Australia's Olyroos have already proven they can pack a punch and are likely to make things difficult for a relatively experienced and more naturally gifted side.