Uzbekistan's Istomin fired 12 aces in rebounding from a dropped first set to overcome Germany's Bjorn Phau 5-7, 7-5, 6-1.
Meanwhile, Japan's Ito moved on to the second round after dumping Roberto Bautista-Agut of Spain 6-3, 1-6, 6-2 at Moscow's Olympic Stadium.
Tunisia's Malek Jaziri had little trouble with American Rajeev Ram, posting a 6-3, 6-1 win. The Czech Republic's Lukas Rosol logged a 7-6 (11-9), 7-6 (7-2) decision over Spaniard Daniel Gimeno-Traver in other Day 1 matches.
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