The West Indies are third in the global T20 rankings, while Sri Lanka languishes at a modest eighth.
All-rounder Shamar Springer is making his debut for West Indies, while Bhanuka Rajapaksa is making a comeback to the Sri Lankan side after 18 months.
"The weather is not so great, so we opted to bowl first," Powell said. "We have been playing some good cricket in the last 15 months, but Sri Lanka are a tough side."
The toss was delayed by 30 minutes due to a wet outfield, and Sri Lankan captain Charith Asalanka said he would also have bowled first due to the weather had he won the toss.
Upbeat hosts Sri Lanka are hoping to continue a run of success after celebrating victories in Test and ODIs, and stretch their victories to T20s as well.
"We have turned things around in Test and ODI cricket, but in T20s we have struggled -- we know the World Cup is two years away, and we are focusing on it," Asalanka said.
Sri Lanka ended a 27-year dry spell by winning an ODI series against India, celebrated a Test victory in England after a decade, and notched their first series win over New Zealand since 2009.
The remaining two T20s against the West Indies, also at Dambulla, will be played on October 15 and 17.
The T20 series will be followed by a three-match ODI series.
Teams
Sri Lanka: Pathum Nissanka, Kusal Mendis (wk), Kusal Perera, Kamindu Mendis, Charith Asalanka (capt), Bhanuka Rajapaksa, Wanindu Hasaranga, Chamindu Wickramasinghe, Maheesh Theekshana, Matheesha Pathirana, Asitha Fernando
West Indies: Evin Lewis, Brandon King, Shai Hope (wk), Roston Chase, Sherfane Rutherford, Rovman Powell (capt), Shamar Springer, Romario Shepherd, Alzarri Joseph, Gudakesh Motie, Shamar Joseph
Umpires: Raveendra Wimalasiri (SRI), Ruchira Palliyaguruge (SRI)
TV Umpire: Lyndon Hannibal (SRI)
Match Referee: Ranjan Madugalle (SRI)
