Ochoa, who took the top spot in the rankings from Annika Sorenstam on Monday, had two eagles, five birdies and four bogeys in windy conditions on the Tres Marias course, the Jack Nicklaus-designed layout that sits 6,300 feet above sea level.
After playing the back nine first, the 10-time LPGA Tour winner eagled the par-5 fifth and eighth holes, hitting a 7-wood from 216 yards to 21 feet on No. 5 and a 9-iron from 160 yards to 27 feet on No. 8.
'It was a day of ups and downs. I was very pleased with the eagles and the bogeys come with the emotion of playing a little aggressively,' said Ochoa, followed by a swarm of fans giving hearty applause on almost every swing.
'We planned to get 5-under each day so I'm very tranquil and happy. It's going according to plan and we hope that will carry on over the following days.'
Prammanasudh, the Fields Open winner in Hawaii in February, had four birdies and an eagle en route to her 67.
'I knew you had to make birdies out there, just with how soft the greens are and you can be really aggressive with your putts,' the Oklahoman said. 'Luckily, I didn't have any mistakes out there and just made some birdies when I had to.'
South Korea's Na On Min matched Ochoa with a 68. The South Korean rookie had eight birdies, a bogey and a double bogey.
Ochoa topped the world rankings Monday with 12.84 points, 0.14 points ahead of Sorenstam. Sorenstam had led the list since the rankings began in February 2006, but has been sidelined by a ruptured disk in her back.
The news was greeted with euphoria south of the border, being splashed across the front of Mexican newspapers and President Felipe Calderon even made a congratulatory call to Ochoa. Rivals such as Prammanasudh say it can be tough playing against Ochoa with such support in her homeland.
'You know it going in that Lorena has a whole entire country behind her. When we go to certain towns, like next week it's my hometown, we just have a city behind us. She's got a whole country. It's phenomenal,' Prammanasudh said. 'If you are paired with her or anywhere around her there's going to be some extra noise.'
However, the pressure has not always been good for Ochoa, who has struggled in Mexico.
In the 2005 Morelia tournament she tied for 16th, while the crown was taken by Sweden's Carin Koch. She also wasn't a factor in the 2005 and 2006 MasterCard Classic tournaments in Huixquilucan, outside Mexico City, and struggled to tie for sixth there this year.
Ochoa said she is learning to play at her best under the spotlight.
'Pressure is always a factor but in this moment, I think it's something positive for me,' she said. 'At first, it was difficult but now I feel more relaxed. I'm very happy playing in front of my public.'
Lisa Fernandes, Silvia Cavalleri, Linda Wessberg and Moira Dunn opened with 69s. Meaghan Francella, the MasterCard Classic winner last month in a playoff with Sorenstam, shot a 73.