Istomin had seven aces in the first set and three in the second of a 7-5, 6-1 win over Paul-Henri Mathieu. Istomin had four breaks and dominated the second half of the match. He took 72 percent, including 16-of-19 serving, of the points in the second set.
Sixth-seeded Robin Haase also had four breaks in a 6-3, 6-2 decision over Simone Bolelli. Haase won nearly half the points off Bolelli's serve.
Paolo Lorenzi outlasted Edouard Roger-Vasselin 3-6, 6-6 (7-2), 6-4 in a match that took more than 2 1/2 hours. Lorenzi earned a second-round match against No. 2-seeded Richard Gasquet, who had a first-round bye.
Defending champion Juan Martin del Potro is the top seed for the tournament.
Swim Week Miami Beach 2014 [PHOTOS]