The second game took just as long, but importantly, Sun got a break.
She got another break two games later to go up 4-0 and it was only after then that Kasatkina found her range, getting both of the breaks back.
The momentum stayed with Kasatkina, as Sun double-faulted in the last two points of the game at 4-4.
But then the fightback began. Sun saved a set point in the next game and when Kasatkina served at 5-6, the Kiwi got another break to take the set.
Sun was broken in the fifth game of the next set and once more later on, sending the match the full distance.
But again, Sun was able to get through a bad patch, breaking Kasatkina in the second game and then confidently holding to go up 3-0.
Although there were some nervous moments, Sun stayed composed and kept hold of the break, to secure one of the biggest victories of her career.
“It’s just like windy Wellington,” Sun smiled afterwards.
“But honestly, I was expecting it beforehand. We’re by the coast and I watched some videos here from last year, and I saw that the conditions were pretty tough.
“So in my mind I had to manage every ball, every shot, just trying to keep moving, keep going and keep fighting.
“One ball goes five metres to the left and then the right. And you can’t keep one shot or one loss of point in the back of your mind.”
Sun has had impressive wins on clay and hard courts over the past year, but grass has clearly become her best surface and she says she’s enjoyed being back on it.
“It’s always great to be able to play on grass.
“It’s a really unique opportunity during the [grass court] season because it’s so short.
“So I always try to enjoy my time the most I can. This is my second year playing in a row, so I haven’t played a lot [on grass], but every opportunity I have is amazing.”