Wellington, playing their fourth game in 14 days, were resolute in defence and although, at times, they missed the creativity of Carlos Hernandez in attack, who is sidelined for up to six weeks with a fractured wrist, they never wilted under pressure.
After winning their past two games 1-0, it was courtesy of second-half goals to Tyler Boyd and Stein Huysegems that gave the Phoenix the edge over the second-placed Wanderers, who were heavy favourites coming in to the match.
The first spell was a stalemate as neither side could find an early goal. Chances were rare and Wanderers marquee man Shinji Ono had the best opportunity of the game but his header hit the crossbar late in the first spell.
Coach Merrick went to his bench at halftime and inserted young gun Matthew Ridenton in to the contest at the expense of Jason Hicks, who had enjoyed an industrious opening 45 minutes.
It marked the 17-year-old Ridenton's second appearance for the Phoenix after he joined the club on a three-year deal in October and he looked sharp during his time on the field.
The deadlock was finally broken in the 62nd minute when Ono delivered a deft touch for Mark Bridge to latch on to and he finished smartly to give the home side the lead.
But their advantage was short-lived as substitute Boyd got his first A-League goal only six minutes later as he prodded home a Huysegems cross as the Phoenix levelled things at 1-1.
Then it was down to Huysegems, who added a brace within six minutes as Wellington secured an unlikely victory, which marked a notable turnaround after they began the season without a win from their opening 10 games.
Merrick was given another injury-related headache from the game though when right back Louis Fenton left the field late in the first half after he took another nasty fall on to his troublesome right shoulder.
Fenton dislocated the shoulder in the pre-season and popped it out during last week's 1-0 win over the Melbourne Heart but it was put straight back in and he was deemed fit to meet the Wanderers.
But when he left the pitch in the 40th minute tonight he looked in severe pain and Merrick will now need to find a new right back as Fenton joined Paul Ifill (ruptured Achilles tendon) and Hernandez in the doctor's office.
Ifill is cooked for the season and Hernandez is out for more than a month, which will put pressure on Merrick's squad and he may be forced to dip in to the transfer market this month.
Fringe All White Michael Boxall, who is usually more comfortable in the centre of defence, came off the bench tonight to replace Fenton and produced a solid shift at right back.
The Phoenix still sit a couple of wins outside of the top six but the wheels are in motion for a mid-season charge that could push them towards the playoffs.
Wellington Phoenix 3 (Tyler Boyd 68, Stein Huysegems 78, 84), Western Sydney Wanderers 1 (Mark Bridge 62). Halftime: 0-0.
- Daniel Richardson of APNZ
