Yamashita, celebrating her 24th birthday on Saturday, led Rio Takeda by three shots after two rounds and was seven clear of a group sharing third place.
While several of the chasing pack did all they could to add pressure before the Japanese duo teed off, much of that effort would have proved irrelevant had the final group shot low themselves.
However, they registered only one birdie between them in their opening 10 holes and Yamashita’s lead was cut to a single stroke when she bogeyed the 10th.
Consecutive birdies at the 11th and 12th restored a cushion, but Yamashita dropped two further shots heading home and it took an incredible long-range putt to save par on the 17th to ensure she would take a solo lead into the final round.
With wind and showers forecast on Sunday, a fascinating conclusion awaits in south Wales.
More to follow.