A nodded far-post header was all it took to separate two very evenly-matched sides on a cool night in Campelltown on which clear-cut chances were at a premium.
Darije Kalezic's men showed tremendous fight in his first competitive fixture in charge, and despite a string of injuries a Phoenix side boasting three Academy players gave as good as they got over two gruelling hours.
It was a well-contested first half with both sides enjoying good patches of possession in midfield.
Phoenix suffered some bad news during warm up with experienced defender Marco Rossi ruled out due to calf injury.
It meant a re-shuffled look to an already threadbare Phoenix line up, with Academy graduates Sarpreet Singh and Liam Wood handed starting places, and the latter immediately took to his new environment well cutting out a dangerous early Wanderers cross.
Roy Krishna was lively out wide right with an early effort on goal, before Steven Lustica drew the first save from either 'keeper in the 12th minute, which Lewis Italiano fielded comfortably.
New signings Goran Paracki and Scott Galloway made their competitive debuts and put in two of the more impressive performances in Phoenix yellow.
The Croatian Paracki looked energetic both in defence and attack, releasing Krishna again, whose opposite number Brendan Hamill became the first man booked as he struggled with the Fijian's pace.
Krishna found himself with a double chance in the 40th minute but resolute defending from the home side kept the score level.
Roly Bonevacia was at the heart of most of Wanderer's attacking moves as he looked to get one over his former club, it was his lofted ball that almost found new striker Riera before the Dutchman blazed over on the stroke of half-time with perhaps the best chance of the first half.
In the second stanza the cat-and-mouse continued, before Italiano provided the save of the evening, with a strong hand denying a point-blank header from Wanderers' skipper Robert Cornthwaite.
Tempers flared after Kearyn Bacchus tangled with Logan Rogerson as Lustica also went into the book. The young striker almost opened his Phoenix account as he rippled the side netting in the 60th minute before he made way for another Academy graduate Tinashe Marowa.
Manyiel Majok entered the fray for the home team and almost found the opener in the 80th minute but for some excellent defence from Tom Doyle, moved to centre back in Rossi's absence.
Alex Rufer made the most of his return to the Phoenix line-up after excellent work from skipper Michael McGlinchey found Marowa, whose layoff was stroked goalwards by the young All White in the 88th minute but it sailed just over.
In a frenetic last few minutes of regulation time both Krishna and Bonevacia went close, with the former Phoenix midfielder's deflected shot looping dangerously past the post.
Two tense periods of extra-time followed where final-third chances were few and far between. Riera threatened in the 112th minute, almost latching on the end of a whipped cross before the former Deportivo La Coruna striker finally broke the deadlock in agonising fashion.
A desperate late lunge from Doyle saw the Phoenix defender booked and with Wanderers camped around the box for the final few moments, it was Riera who rose highest at the far post to nod home.
Post-game Phoenix coach Darije Kalezic was circumspect in defeat, but took some satisfaction in the defensive progress already made.
“Every loss is disappointing, especially tonight," said Kalezic. "But my feeling is a positive feeling because we performed well tonight, we played a good game - good structure, with good organisation and high discipline.”
“We had many, many [injury problems] like you said … but the young players performed well tonight, which makes me satisfied.”
“We start with a new age, a new period in Wellington Phoenix. We have a vision of working, which is not to win the game tonight – of course we want to win the game – but the result doesn’t have an influence on the vision of what we want to implement at the club.”
This performance doesn’t give us any [insight] into the future, this performance tonight confirms only that how we’ve worked until now is right, and gives us trust to go in the same way to develop the team for next two and a half months ahead of the [A-League season].”