Willett, who has struggled for form and fitness since winning at Augusta National, fell out of the top 450 in the world rankings earlier this year, but was in great form at the Earth Course, where he shot 68 or better on all four days.
The world number 96, who already has two top-five finishes on the US PGA Tour this season, saw overnight challengers including Americans Matt Kuchar and Keegan Bradley fade away and was never seriously threatened.
World No. 96 Ancer, who already has two top five finishes on the US PGA Tour this season, started two shots off the leaders but made an incredible run of five birdies at the sixth to 10th holes to take control.
An Byeong-hun and local hope Max McCardle are one shot clear at eight-under-par from in-form Matt Kuchar.
The world number seven plans to concentrate on playing in the United States, especially with the changes that have been made to the 2019 world golf schedule.
The 40-year-old American, who had started the final round with a four-shot lead, squeezed home ahead of New Zealand's Danny Lee after posting a two-under-par 69.
The 136th-ranked Mexican beat off world number one Ariya Jutanugarn of Thailand and a late challenge from France's Celine Boutier.
Star attraction Rory McIlroy from Northern Ireland could manage only a 71, leaving him eight shots adrift of the 2017 Masters champion.
The Mexican's 183-yard hole-in-one helped her leapfrog world number one Ariya Jutanugarn.
Finn Mikko Korhonen, Mike Lorenzo-Vera from France and South African Charl Schwartzel share second place.
Thidapa shot a four-under-par 68 on the island of Hainan to take a one-shot lead from Thailand's world number one Ariya Jutanugarn and Canada's Alena Sharp.
The Englishman’s victory along the shores of the Mediterranean is the first time in his now 19 pro career wins around the globe he’s successfully defended a title.
Hataoka became the first home player in seven years to win the Japan title, firing seven birdies with two bogeys to finish 14-under par at Seta Golf Club in Shiga.
Lee, who was tied for fifth in the first round, displayed a steady performance with precision shots, while first-round leader Ryu So-yeon sank to be tied 13th with three bogeys.
The 25-year-old began and ended his round in Belek with birdies to head the 78-player field by a shot in ideal scoring conditions on the Regnum Caya course.