Chivas USA coach Preki also gave starting assignments to a lot of his backups, and in temperatures exceeding 80 degrees the visitor created the first early scoring chance when defender Shavar Thomas hit the post with a header.
Injuries affected both teams in the early going, with Fire defender C.J. Brown and Chivas USA right back Mariano Trujillo hobbling off after a quarter of an hour. Chicago got its goal when Patrick Nyarko stole a back pass and hit a low cross that caromed off two defenders to Mapp, who connected with a rising left-footer from just inside the penalty area.
Austin Washington denied Chivas USA an equalizer early in the second half when he tucked in behind goalkeeper Jon Busch, who'd been rounded by Maykel Galindo, and knocked away the Cuban striker's shot on frame.
A melee broke out in the 80th minute after Chicago attacked the Chivas USA goal rather than kick the ball out of play as defender Ante Jazic lay grounded by an injury. After goalkeeper Zach Thornton saved Marco Pappa's shot, bickering broke out and the officials needed a minute to resume play.
After a scoreless first half in the second game of the doubleheader, substitute Jose Reyes scored twice for San Luis. Midfielder Braulio Luna connected with a free kick early in the second half, and Reyes got his first goal from a Luna assist three minutes later. Juan Pablo Garcia's goal for UNL in the 74th minute preceded a second by Reyes by only two minutes.
A Group B doubleheader is on tap for Wednesday at Gillette Stadium. The Revs (1-0-0) and Kansas City (0-0-1) play at 7 p.m. ET (TeleFutura); Atlas (0-0-1) and Santos Laguna (0-1-0) meet at 9 p.m. (Telefutura). New England rallied last weekend to thrash Santos, 4-2, and the Wizards played Atlas to a 0-0 tie.
Group play concludes this weekend. The top two teams in each group advance to the semifinals to be played next month.
June 23 in Bridgeview, Ill.
Chicago 1, Chivas USA 0
Goal: Mapp 35.
Chicago - Busch; Brown (Prideaux, 16), Conde, Washington, Segares, Mapp (Pappa, 69), Pause, Husidic (Rolfe, 89), Banner, Dimitrov (Ward, 69), Nyarko.
Chivas USA - Thornton, Talley, Thomas, Jazic, Trujillo (Curtin, 21), Victorine, Flores (Chijindu, 46), Nagamura (Marsch, 46), Lahoud (Galindo, 46), Stepanovic, Eskandarian.
Referee: Mark Geiger.
San Luis 3, UNL 1.
Goals: Luna 53, Jose Reyes 56, 76; Garcia 74.
San Luis - Trejo, Mojaras, A. Garcia, Mascorro, de la Cruz, A. Moreno (Salinas, 86), M. Moreno, Torres, Luna, Lojero (Reyes, 46), Palacios.
UNL - Palos; Valencia (Viniegra, 52), A. Pulido, Jimenez (De Leon, 71), Cabrera, Duenas (Acuna, 58), A. Pulido, Saavedra, Rivas (Cerda, 57), Garcia, Molina.
Referee: Ricardo Arellano.