Chasing 159 for victory at the R Premadasa Stadium, the Indian juniors rode on a solid 89-run opening stand between Sundar (56) and Rishabh Pant (35) to achieve the target in 33.5 overs.
Left-arm spinner Damitha Silva did pick up three wickets to cause a bit of a stutter in the Indian chase but Ricky Bhui (29 not out) made sure the visitors pocket the summit clash with ease and remain unbeaten in the tournament.
Wicketkeeper-batsman Vishad Randika top-scored with 58 runs, while left-arm pacer Khaleel Ahmed bagged three wickets.
Pace spearhead Avesh Khan, medium-pacer Shubham Mavi and left-arm spinner Mayank Dagar picked two wickets each.
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The third team in the tri-nation event, England U-19 remained winless in the four matches they played.
Brief Scores:
Sri Lanka Under-19 158 all out in 47.2 overs (Vishad Randika 58, Damitha Silva 23; Khaleel Ahmed 3-29)
India Under-19 159/5 in 33.5 overs (Washington Sundar 56, Rishabh Pant 35; Damitha Silva 3-18).