Opener Simmons hit 67 from 51 balls, including seven boundaries and two sixes, after the West Indies won the toss and elected to bat. He got good support from Andre Russell, who plundered four sixes in a 14-ball 35, and Keiron Pollard (34).
Thomas bowled short and with pace, taking three wickets in the second over to have Sri Lanka reeling at 17-3 after dismissing Avishka Fernando (7), Shehan Jayasuriya (0) and Kusal Mendis (0) within four deliveries.
Despite a half-century from Kusal Perera, who scored 66 from 38 balls, Sri Lanka couldn't get close after Thomas collected his five-wicket haul inside the first power play by removing Angelo Mathews (10) and bowling Dasun Shanaka (2). He returned 5-28 — figures inflated by seven wides — and Sri Lanka was eventually all out for 171 in 19.1 overs.
Sri Lanka will be aiming to square the two-game series when the second T20 is played Friday.