In the second leg Tuesday at Juan Ramon Loubriel Stadium, it must win by two goals to advance automatically, due to the Islanders' away goal.
Kendall Jagdeosingh's goal midway through the second half propelled the USL-1 team to yet another Concacaf triumph against top-tier opposition.
In the 2008-09 competition, the Islanders beat Alajeula of Costa Rica in the preliminary round, and after finishing second in its group, toppled Honduran club Marathon in the quarterfinals. They lost in the semifinals to Cruz Azul of Mexico on penalties after tying, 3-3, on aggregate.
TFC cranked out a 17-6 edge in shots, but five saves by former Crew goalkeeper Bill Gaudette and a couple of crucial misses left the Reds frustrated. Dwayne De Rosario headed wide from a good cross by Chad Barrett in the 26th minute, and a sharp De Rosario pass narrowly missed the feet of Ali Gerba in front of goal.
Gerba threatened again early in the second half when midfielder Sam Cronin, just back from his U.S. appearances in the Gold Cup, steered him a ball he drilled on frame that Gaudette saved. Cronin nearly scored himself near the hour mark but his deflected shot spun wide of the post.
The Islanders scored when goalie Stefan Frei came out erratically to collect a ball. When he failed to control it, Jagdeosingh did, and fired a shot past the rookie keeper.
A flurry of chances late in the game produced some furious goalmouth action but that was all. Gerba banged a ball into the net with a flag up for offside, Marvell Wynne shot wide, and Gaudette just managed to keep a Danny Dichio header out of the net.
One more preliminary-round match involving an MLS team is scheduled. New York Red Bulls play W Connection of Trinidad & Tobago Thursday (8p.m. ET, Fox Soccer Channel) at Manny Ramjohn Stadium in Marabella.
Concacaf Champions League
Preliminary round, first leg
July 29 in Toronto
Toronto FC 0, Puerto Rico Islanders 1
Goal: Jagdeosingh 67.
Toronto FC -- Frei, Wynne, Garcia, Gomez, Brennan, Vitti, Cronin, Robinson (Dichio 86), De Rosario, Barrett (White 74), Gerba.
Puerto Rico Islanders -- Gaudette, Arrieta, Henry, Krause, Jones, Steele (Veris 83), Delgado, Gargan, Gbandi (Rivera 91+), Marcelin, Jagdeosingh (Addlery 74).
Referee: Hugo Cruz.