The 24-year-old Ernst, who clinched her maiden victory on the circuit with a play-off win over South Korea’s Kim In-kyung at the 2014 Portland Classic, carded a flawless five-under-par 67 for a 10-under total of 206.
Second-round leader Sarah Jane Smith of Australia slipped back with a level-par 72 at Club de Golf Mexico to end the day in a tie for second with Spaniard Carlota Ciganda (68).
American Angela Stanford (69), Karine Icher of France (70) and South Korean Hur Mi-jung (71) were a further stroke back at eight-under.
“I have been hitting my drive pretty well, and I have had a lot of good looks at birdie this week,” Ernst told reporters after shooting a second successive 67.
“Today I still made five birdies so I have just made it very easy when I haven’t been sure about myself. I have been playing very well this week.
“I have been hitting the ball well and putting wise… it seems like the speed is really good or the line is barely off.”
Ciganda, who held a share of the first-round lead, made a sizzling start on Saturday with four consecutive birdies from the par-five second before losing momentum.
“I started playing very good ... I felt really comfortable and then I slowed down a little bit,” said the Spaniard, who won her first LPGA title at the KEB Hana Bank Championship in South Korea last month.
“But I’m still playing good – hitting lots fairways, greens. I’m happy with the round.”
Smith, an LPGA rookie in 2006 whose best finish was a tie for second at the 2014 Kingsmill Championship, conceded she had felt a little shaky holding her first 36-hole lead on the circuit.
“I was pretty nervous most of the day,” she said. “I wouldn’t say I handled it well but I scrambled well, so that should give me some confidence for tomorrow.”
Original source: Ernst seizes one-stroke lead in Mexico