Ty Rattie had two goals and an assist for Portland (1-1), Taylor Leier had a goal and two assists, while Oliver Bjorkstrand added a goal and an assist.
Mac Carruth made 25 saves after allowing all seven goals in Saturday's 7-4 loss to the Halifax Mooseheads as the Western Hockey League champions picked up their first win at the tournament.
All four teams at the tournament sit at 1-1, which guarantees a tiebreaker game on Thursday.
Bo Horvat and Alex Broadhurst replied with a goal and an assist each for London, while Scott Harrington also scored. Anthony Stolarz allowed five goals on 26 shots before being replaced by Jake Patterson. The Knights' backup finished with five saves.
Portland defenceman Seth Jones, the No. 1 ranked North American skater according to NHL Central Scouting ahead of next month's draft, had a relatively quiet night for the Winterhawks, but still finished with a plus-4 rating.
After London tied the game 3-3 early in the third period, Pouliot, a first-round pick of the Pittsburgh Penguins in 2012, ripped a shot off Stolarz's shoulder and in on a Portland power play at 6:20. De Leo ended the London netminder's night at 6:43 by sliding a shot home from the slot.
The Knights had a couple of chances to get closer as the period wore on but Carruth shut the door.
Down 3-2 after two periods, the Knights tied it early in the third when London forward Brett Welychka's attempted deflection in front of Carruth fell to Broadhurst at the side of the Portland goal for an easy tap in.
The Winterhawks netminder kept the score tied with a stop of Josh Anderson on a partial breakaway a couple minutes later before Pouliot and De Leo struck in quick succession and Rattie scored into an empty net.
With Portland leading 1-0 after a sleepy first period, the floodgates opened in the second. Bjorkstrand doubled the Winterhawks' lead 2:17 into the second after wheeling out from behind the London net and snapping a shot shortside on Stolarz.
Rattie then stretched the advantage to three with a goal that was a carbon copy of his effort against Halifax on Saturday night. The St. Louis Blues draft pick moved into the London zone and toe dragged the puck around a Knights defender before slipping a backhand past Stolarz at 10:23 while falling to the ice.
Needing a spark, the Ontario Hockey League champions responded just 15 seconds later as Harrington moved into the high slot and wired a shot into the top corner past Carruth.
Horvat scored the game's second highlight-reel goal at 12:30 on the power play. The 15th-ranked North American skater ahead of the draft took a slick between-the-legs pass from Max Domi right in front of Carruth before burying a shot past the Portland goalie.
Horvat nearly tied the score with four minutes left in the period but his shot rang off the post. Carruth then stoned both Domi and Horvat on back-to-back rushes on another London power play to keep the Winterhawks up by one heading to the third.
The crowd of 7,775 at the Credit Union Centre seemed half asleep in the early going and the two teams didn't do much to help wake them out of their slumber.
Coming off the high of the host Saskatoon Blades' exciting 5-2 victory over Halifax on Sunday night, the half-empty arena finally came to life at 16:53 of the first period when Leier deflected a shot off the sideboards from Taylor Wotherspoon past Stolarz for the only action of note in a drab opening 20 minutes.
Notes: The Knights are back in action Tuesday against the Halifax Mooseheads (1-1) in the final round-robin game for both teams. Portland wraps up its schedule on Wednesday against the Saskatoon (1-1). ... The Winterhawks won the Memorial Cup in 1983 and 1998, while the Knights were victorious as tournament hosts in 2005. London lost last year's final 2-1 in overtime to the host Shawinigan Cataractes.
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