Tardy, who eagled the par-5 sixth on her way to a four-under-par round, stands at seven-under for the tournament and has a two-stroke advantage over compatriot Allisen Corpuz and South Korea's Kim Hyo-joo.
Tardy, who finally earned her card for the LPGA this year after competing on the developmental Epson Tour, has struggled in her rookie year, missing five cuts and only once making the top 25.
After she followed up her first round 69 with another strong showing on the challenging California course, she admitted that she almost "forgot" to apply for the qualification event in Minnesota, a gruelling 36 holes in a single day.
"I don't think anybody likes playing in the US Open qualifiers. I haven't played in one for a while because I don't like them," she said.
"So I think it was just a little bit of procrastination. I knew there was going to be a site that I could sign up. I didn't know which one to pick. You have a ton.
"My mom could tell me, 'You need to sign up, you need to sign up' and then I just never did. That's just kind of how it happened. Big ol' procrastination."
Still, being familiar with Pebble Beach, having played a number of events on the famous course over the years, Tardy said that while others might be surprised at her position on the leaderboard, she certainly believed.
"Other people maybe not, but I've always believed in myself to win any tournament that I enter. I'm good enough to be here, and I've been on the LPGA, so I feel like I definitely have the game to hang with the best in the world," she said.
"It feels great. I haven't performed great in the previous majors this year, so just to finally be able to throw some good rounds in and finally have everything come together, I feel like I've had really good ball-striking days and then terrible putting days, and then I'll have a great putting day but terrible ball-striking day.
"So it's finally coming together and meshing well, and it just happens at the right time."
Corpuz, who finished fifth in another major, the Chevron Championship in Texas in April where she shared the lead going into the final round, bogeyed the final hole, ending with a two-under 70.
The Honolulu-born 25-year-old, shared 15th at the Women's PGA Championship and said the test for her now was keeping her focus while in the race this weekend.
"I honestly still need to get a little more comfortable, I think, in contention," she said.
"That's really been the focus this year, just really trying to put myself into that spot and then hopefully learn how to convert as it keeps happening."
Kim, who shared the first-round lead, shot a one-under, 71 after bogeying eight and nine, her final two holes.
"Of course I have some regrets, but what can I do? It's over, right? I'm still on the leaderboard, so I'll just continue to work very hard and I have a chance to be a champion," she said.
Kim is looking for her second major title after winning the Evian Championship in 2014.
Another South Korean, Ryu Hae-ran, was two shots behind while Japan's Nasa Hataoka and Ireland's Leona Maguire were both on one-under after shooting two-over 74.