The Scot’s four-under-par 68 at Clearwater Golf Club saw her end the opening round sitting joint-third, two shots behind Nicole Broche Larsen as the Dane took route 66 to set the pace.
Pretswell, who finished sixth in the same event 12 months ago, was out in level-par after starting at the tenth before picking up shots at the second, fifth, seventh and eighth on her inward journey.
It was an excellent start to the new Ladies European Tour season for the 26-year-old Lanark-born player as she bids to build on finishing a splendid 14th on the money-list in 2015.
“I’m happy to have the new season underway after a couple months since our last event in Dubai,” said Pretswell. “I enjoyed some time off over the winter before working hard with my coach Alan Murdoch.
“I’m really happy with my first round today as I played very solid in some tough, windy conditions this afternoon.
“My ball striking was the best it has been, which made life easier in the wind, and I was very tidy on the greens, which was good to see after some hard work on my putting over the winter.”
The former Curtis Cup player added: “I am excited to be back competing and really enjoying playing golf! New Zealand is my favourite stop on tour so it’s good to start the season here.
“I have little things that I want to keep improving and I hope that if I work on them I will see further progress.”
Kelsey MacDonald, Kylie Walker and Gemma Dryburgh all opened with 73s to sit just inside the projected cut mark, but But Heather MacRae is two below the line on 75.
Ko, bidding to win the event for a third time, mixed five birdies with two bogeys as Broche Larsen carved out a one-shot lead over Frenchwoman Justine Dreher.
Another French player, former Scottish Ladies Open champion Gwladys Nocera, is sitting alongside Pretswell.
Back on the PGA Tour after his European Tour appearance in Abu Dhabi last month, Russell Knox got off to a slow start in the AT&T Pebble Beach Pro-Am.
The 30-year-old Scot carded a one-under-par 71 at Pebble Beach, where Swede Freddie Jacobsen had the day’s best score with a 65.
American Chez Reavie leads overall after an eight-under 63 at Monterey Peninsula, while world No 1 Jordan Spieth opened with a one-under 71 at Spyglass Hills to sit five shots behind Justin Rose there.
Cawder’s Jamie Savage heads into the final round of the African Amateur Stroke-Play Championship at Leopard Creek today trying to make up a five-shot deficit on home player Albert Venter.
While Venter, the first-round pacesetter before being joined by Savage and three others at the halfway stage, regained top spot with a second 65 in three rounds to sit on 13-under, Savage signed for a 70 to sit joint-third.
Back-to-back bogeys at the eighth and ninth didn’t help the Scot in his title bid but he came home in three-under to stay in contention.
Barassie’s Jack McDonald carded a six-under 66 to move into a share of fifth on five-under alongside Kirkhill’s Craig Ross (70).
Having also signed for a two-under effort to sit three-under, Australian Amateur champion Connor Syme (Drumoig Golf Centre) completes a quartet of Scots in the top 10.
Chris Robb was the only Scot from four starters to survive the cut in the Pro Golf Tour’s Tazegzout Open in Agadir.
Even then, the 2014 Scottish Amateur champion made it by the skin of his on two-over after rounds of 74 and 72.
There were early exits for Ross Cameron (73-75), Sam Kiloh (76-77) and Zander Culverwell (79-81).
Jay Taylor, who played at Marriott Dalmahoy as an amateur but is now attached to PGA Portugal, is just two shots off the lead at the halfway stage of the Algarve Pro Tour’s 36-hole Boavista Classic at Boavista Golf Resort and Spa, near Lagos.
Taylor’s flawless three-under 68 (33-35) left him sitting joint-third, within striking distance of the pacesetter, Englishman James Adams.
Paul Lawrie Golf Centre pair Craig Lawrie (33-36) and Greig Hutcheon (32-37) are joint seventh on 69.