The second-seeded Fish, who was the runner-up in last year's event, breezed by Australian lucky-loser Matthew Ebden 6-2, 6-0 in a 53-minute blow-out on the hard courts at Rexall Center.
His next opponent will be Argentina's Juan Monaco, who posted a 7-5, 6-4 second-round win over Canadian wild card Vasek Pospisil Tuesday.
The eighth-seeded Isner, meanwhile, bested Spain's Pablo Andujar, 7-6 (7-5), 7-5 in a second-round evening match-up.
Haas -- who has appeared in three finals in the last two months -- emerged with a 6-2, 6-7 (11-13), 6-3 first-round triumph over David Nalbandian of Argentina in a match that lasted 2 hours, 22 minutes.
Canada's Milos Raonic, seeded 16th, won his second-round meeting with Serbian Viktor Troicki, 6-3, 6-4, during the night session.
Also logging first-round wins were last week's Los Angeles titlist Sam Querrey of the United States, Czech Radek Stepanek, Spaniard Marcel Granollers, Russian Mikhail Youzhny, Italian Fabio Fognini and Kazakhstan's Mikhail Kukushkin and Frenchman Julien Benneteau.