Striker Roen Nelson was again the hero for Portmore, cancelling Aphas Bernard's strike before adding another in the second half after Anthony Bennett made it 2-1.
Portrmore, with a game in hand against hot Village United, are locked on 53 points with Tivoli, who drew 0-0 at Sporting Central.
Battle for placingThe battle for third and fourth spot behind the top two teams also intensified as Waterhouse edged Boys' Town
1-0 at Drewsland to pull within a point of their rivals.
It was Waterhouse's first win in seven games and Boys' Town's first loss of the third round, keeping them on 43 points in third, one ahead of their stalking rivals.
Both remained clear of defending champions Harbour View, who continue to show inconsistent form, drawing 0-0 at Jarrett Park against Seba.
However, the Stars of the East remain unperturbed in fifth place on 38 points as their closest pursuers, St Georges, were ambushed 3-1 at Reno.
St Georges, hanging on to sixth place on goals scored, are now joined on 35 points by two teams, Seba and Village, who knocked over Arnett Gardens 3-1 at the Tony Spaulding Sports Complex to leap from ninth place to
seventh.
With four third-round games left to decide the top six teams to play for the title and bottom six to avoid relegation in the fourth round, Village's home game in hand against Portmore will be crucial for the men from Trelawny.
At the foot of the standings, August Town remained anchored on 26 points whereas Reno have skipped up to 30, one off Arnett Gardens', who remain on slippery ground ahead of the fourth round.
Waterhouse and Boys' Town's clash at Drewsland started out promising and was still an entertaining match when both teams were reduced to 10 men after 20 minutes following the ejection of Marco McDonald and Daemion Benjamin.
The homesters had the better of early attacking play but selfishness resulted in Roberto Fletcher and Kenardo Forbes attempting impossible shots from acute angles on either side of goal with Jomo Gordon standing almost on the goal line in both instances, the 29th and 30th minutes.
Boys' Town had their chances before Jonathan Williams netted for Waterhouse in the 39th as George Vernal's whistling 27th minute free kick would have left Richard McCallum a spectator spare a couple inches.
Williams got what turned out to be the match-winner six minutes before the break, hitting Leon Strickland's pass over Kirk Porter.
The second half broke down into a scrappy, card-littered affair as both teams lost shape badly.
Waterhouse got a handful of chances throughout the half, especially Gordon who brought out a fine 68th minute save from a diving Porter.