Imam-ul-Haq became the third youngest player to score a century on his one-day international debut and Hasan Ali grabbed 5-34 as Pakistan hammered Sri Lanka by seven wickets to secure a series victory on Wednesday.

Pakistan took an unassailable 3-0 lead in the five-match series, with Sri Lanka's losing streak in ODIs now stretching to 10 matches.

The 21-year-old Imam is the nephew of Inzamam-ul-Haq, the former captain and Pakistan's current chief selector. He hit 100 off 125 balls with two sixes and five fours to help Pakistan cruise to 209-3 in 42.3 overs.

Sri Lanka captain Upul Tharanga top-scored with 61 after winning the toss and opting to bat first, but his team's batting woes in the series continued before it was finally dismissed for 208 in 48.2 overs.

Tharanga and Niroshan Dickwella (18) had provided a solid opening stand of 59 off 65 balls before Hasan clean bowled Dickwella.

Dinesh Chandimal struggled for 49 balls to score 19 before Sri Lanka collapsed midway through its innings as Pakistan's spinners, Shadab Khan (2-37) and Mohammad Hafeez, tied the batsmen down.

Shadab struck in successive overs by having Chandimal trapped lbw and then Tharanga holed out at deep midwicket off a short delivery as Sri Lanka slipped to 112-3.

Ali hastened the collapse when Chamara Kapugedera one of two changes Sri Lanka made after losing the first two matches chipped an easy return catch.

Ali combined with captain and wicketkeeper Sarfraz Ahmed to get Jeffrey Vandersay, Akila Dananjaya and Dushmantha Chameera all caught behind as Sri Lanka struggled after losing Tharanga in the 28th over.

Ali became the quickest Pakistani bowler to complete 50 wickets in ODIs, claiming the feat in 24 matches, three fewer than the previous record set by Waqar Younis.

Imam, replacing out-of-form Ahmed Shehzad at the top order, dominated two productive stands — adding 78 runs with Fakhar Zaman (29) and further 66 with Babar Azam (30).

Imam looked confident from the onset when he smashed seamer Lahiru Gamage for a six over point.

Wicketkeeper Dickwella twice dropped Imam, on 29 and 89, before finally catching him down the leg side.

Pakistan's Salim Elahi is the youngest century scorer on debut, also against Sri Lanka, as an 18-year-old in 1995. Mark Chapman of Hong Kong is the second-youngest at 21 years and 142 days, when he managed the feat against the United Arab Emirates in 2015. Imam will be 22 in December.

The remaining two matches of the series will be played at Sharjah on Friday and next Monday.