Right-winger Jaiden Ibbetson put Wainui 1-0 up after 15 minutes.
But Thistle Reserves started the second half with renewed vigour.
In the 47th minute, right-flank midfielder Merlin Parsons made a good run down the wing and crossed the ball into the goalmouth.
The danger was beaten away only as far as Thistle striker Hammy Il-Jammi, who blasted the ball into the net from 15 metres to equalise.
Thistle had lost the services of central midfielder Ashley McMillan at halftime. He had put in a strong first-half shift, but had to leave early to join the Pacific Premiership squad, for whom he was a substitute.
The Reserves had others who stepped up, though. Their man of the match was central midfielder Charlie Morris, while Andre Riley ran him close for the accolade with a lively performance at leftback.
Team manager-coach Ron Young said goalkeeper Gabrile Garnica and centreback Reece Brew also had standout games keeping the Wainui attack at bay . . . until left-winger Steffan John grabbed the winner in the last 10 minutes.
Wainui coach Smith said Ibbetson and John — both new to the club this season — were neck and neck in the race for the Wainui golden boot award. They had nine goals each so far.
Ibbetson “worked his socks off”, but John was more of a predatory type of striker, whose contributions were less noticeable until he popped up to score a goal . . . “like Robbie Fowler”, Smith said.
Jamie Gallacher and Mal Furlan did sterling work in the heart of the defence, while former centreback Dan Willock shone as a targetman up front.
“That was one of our hardest games of he season,” Smith said.
“They're a nice footballing team.”
Smith said they needed Thistle Massive to do them a favour by beating United this Saturday in the Eastern League curtain-raiser to the Pacific Premiership derby between Thistle and United at Childers Road Reserve.
But he acknowledged United were now favourites to win Eastern League 1.
“This is a good platform for next year,” Smith said.
“We're still getting 20 to training. For Wainui, that's unheard of at this time of year.”
The club is enjoying a purple patch. While the Sharks are second in the first division, Coates Associates Wainui Demons are second and Wainui Sports Club are fourth in the 10-team second division.
Four different scorers got the goals for United against Boys' High, and one of them struck for Boys' High as well.
United stalwart centreback Jonathan Purcell stuck his foot in the way of a shot from Boys' High player Matt McNeil, managing only to wrongfoot keeper Al Knight and give Boys' High an early lead. It should be noted, though, that Boys' High coach Sebastian Itman credits McNeil with the goal.
If he felt he needed to, Purcell redeemed himself about 30 minutes into the game when he got on the end of a flick-on from a Matt McFatter long throw. McFatter had trotted from his leftback spot to the right wing — a regular occurrence apparently — to hurl one of his missiles into the danger area. The ball was flicked on at the near post and Purcell was on hand at the far post to bundle it over the line.
Ten minutes into the second half, the deadlock was broken, and within 10 minutes more the score was 4-1. Stu Cranswick, Craig Christophers and Aubrey Yates got the second-half goals.
United coach Dean Wrigley said outstanding contributions came from Andrew Bristow in a holding midfield role, Christophers in central midfield with his experience and encouraging chat, and Sam Royston for his pacy play on the right wing and willingness to take on more defensive duties as United pushed for more goals.
Knight, in goal, rightback Ben Hansen, centrebacks Purcell and Jimmy Holden, and leftback McFatter were “pretty solid”. Knight made two good saves late in the match that kept Boys' High from getting back into the match.
For Boys' High, Toby Roseman, Sam Dempster and Adam Simpson were their best on the day.
Thistle Massive did enough to get the win against Lytton, coach John Stirton said.
Ten minutes into the game, Lytton went ahead through Cole Devonport's goal. Thistle striker Nic Somerton equalised from the penalty spot five minutes before the break.
In the second half, striker Matias Pincini grabbed two goals to make the game safe for Thistle Massive.
Stirton's man of the match was defensive midfielder Fraser Hopkins. Others to impress were striker Matt Smith, midfielder Kane Stirton and defenders Chris Spurr and Stefan Faber.
Lytton coach Craig Christophers said everyone played well in a good team effort, but Jamil Dos Santos and Toby White put in standout performances.