The passionately supported South Coast side, who have four Aussies on their books, were left hugely enthused on Saturday after their latest recruit, snapped up from Sydney FC in June, capped a fine display with his 38th-minute goal.
The 21-year-old attacking midfielder's all-action display, topped by his well-taken bending strike from the edge of the box after a rebound, left Segecic beaming: "To be able to get a goal with three points on debut was an unbelievable start.
"I was told on the Friday that I was starting the game, which was great to hear and I couldn't ask for much more."
Pompey's manager John Mousinho purred that he wouldn't have fancied having to face the busy youngster.
"Segecic was great, not just to get the winning goal but his work rate was superb as well," Mousinho said.
"His pressing was great and he picked himself up into some very nice pockets at times, and he took his goal really well, so I was really impressed with him on his debut."
After showing plenty of creativity as well as application in the No.10 shirt at Oxford's Kassam Stadium, the famously passionate Pompey fans gave Segecic a rousing reception when he came off in the dying seconds to be replaced by fellow young Aussie Thomas Waddingham.
"To go over and celebrate with the fans at the final whistle was amazing," added Segecic, who's hoping a big season in England's second division will help him nail down a place in the Socceroos' World Cup finals squad.
He scored 13 goals for the Sky Blues last season, enough to make him the A-League Men's top scorer alongside Archie Goodwin.
Portsmouth hold great store by their young Australian talent, with Socceroo Hayden Matthews, an unused sub on Saturday, as well as national under-23 defender Jacob Farrell also on their books.
With teams in England's second tier dreaming of starting their push for the Premier League in winning style, ambitious newly promoted Wrexham got a rude awakening at Southampton as they went down 2-1 after two late goals.
Wrexham, playing non-league soccer just three years ago until their spectacular rise began under celebrity co-owners Ryan Reynolds and Rob McElhenney, led at halftime through a Josh Windass penalty.
But substitute Ryan Manning, with a stunning free kick, and a Jack Stephens winner six minutes into stoppage time was hardly the sort of Hollywood ending Wrexham have become used to.
Ruben Selles' tenure as Sheffield United boss got off to a nightmare start with a 4-1 home defeat by Bristol City, while Harvey Knibbs scored five minutes into stoppage time as Charlton marked their return to the Championship with a 1-0 win over Watford.
With agencies
