Park and Choi each posted 5-under-par 66s Friday in the first round of the ShopRite LPGA Classic in Galloway, N.J. They shared the lead among the morning wave.
While the Americans are off to a sizzling start this season, with seven victories among the first 12 LPGA events staged, the South Koreans have yet to post their first LPGA title of 2014.
The last time the South Koreans failed to post a win in the first dozen LPGA events was 2008, but they finished with a flurry, winning nine titles.
National allegiances have never been more relevant on the tour, with the LPGA preparing for its new Olympic-style team competition, the International Crown, at Caves Valley outside Baltimore July 24-27.
The South Koreans held the No. 1 seed for that competition until the Americans took it away in the final week of qualifying back in April.
Park has held the Rolex No. 1 ranking for 59 consecutive weeks.
The Americans, however, are looking to take that, with No. 2 Stacy Lewis within striking distance.
Lewis can take the top ranking this week with a finish of ninth or better, depending upon what Park and No. 3 Lydia Ko do.