Sri Lanka beat New Zealand in rain-affected ODI to take series lead
Kusal struck a career-best 143 and Avishka hit 100 as Sri Lanka posted a commanding 324-5 in 49.2 overs before rain stopped play.
New Zealand, chasing a revised target of 221 off 27 overs, were restricted to 175-9 in the day-night game played at Dambulla.
The Kiwis started off confidently but then lost five wickets in less than five overs for the addition of just 22 runs.
It was Maheesh Theekshana who triggered the collapse with the wickets of openers Will Young and Tim Robinson, who shared an 88-run stand off just 79 balls.
Robinson fell to a sharp stumping by Kusal, and four balls later, Theekshana bowled Young to swing the momentum towards Sri Lanka.
From there on, it was downhill all the way for the visitors as the rest of the batsmen failed to live up to the task.
"When you lose two set guys, it can be difficult on these wickets," said Kiwi skipper Mitchell Santner.
"We tried to keep firing shots, but if you lose wickets in clumps, it's a challenge."
Earlier, Kusal and Avishka stitched together a 206-run stand off 215 balls for the highest second-wicket partnership in ODIs for Sri Lanka against New Zealand.
Their effort eclipsed a 21-year-old record held by Sanath Jayasuriya and Hashan Tillakaratne, who added 170 against the Black Caps in the 2003 World Cup in Bloemfontein.
The stand was also the highest partnership for any wicket at Dambulla in ODIs.
"I thought it will be tough for us (with the wet ball) and it also didn't turn too much," said Sri Lankan captain Charith Asalanka.
"Avishka and Kusal batted really well and the best thing was that both of them scored hundreds."
For Avishka, it was his first century since 2021, breaking a dry spell after a string of low scores.
Avishka's 115-ball knock, including nine fours and two sixes, came to an end when he was caught while attempting a sweep shot off Ish Sodhi. It was his fourth ODI hundred.
Kusal's century was also his fourth in ODIs and first this year.
His knock was the highest individual score at Dambulla, surpassing India's Shikhar Dhawan's 132 in 2017.
A cameo 40-run knock by Asalanka off just 28 balls capped a fine day with the bat for the islanders.
It was New Zealand paceman Jacob Duffy who dismissed Kusal, caught at extra cover by Henry Nicholls.
Kusal's brisk 128-ball knock was studded with 17 fours and two sixes.
Sri Lanka's batters seized control after Asalanka won the toss and opted to bat first.
Opener Pathum Nissanka was dismissed early for 12, but Kusal and Avishka made the most of the conditions.
The New Zealand spinners struggled to rein in the Sri Lankan batsmen, who kept the scoreboard ticking at over 6.5 runs per over.
Rain halted play just one ball into Sri Lanka's innings, causing a 39-minute delay. Downpours returned with four balls left in the Sri Lankan innings.
The ODI action will next move to Pallekele where the remaining two games of the series will be played on Sunday and Tuesday.