The No.5-seeded Cirstea struggled early in both sets - she faced a point for a 3-1 deficit in the first set and three points for a 3-0 deficit in the second set - but each time she bounced back and just got better and better, eventually needing just an hour and 16 minutes to move through to the second round.
"I'm really happy with the way the match went," Cirstea said. "It was my first match back on the hardcourts and she's a really great player, and the last time we played in Rome I lost quite badly - so going into this match I knew it wouldn't be easy, and I'm quite happy with my overall performance, especially since, like I said, this is the first tournament back on hardcourts for everybody."
What was Cirstea most pleased about in the match? "I was down 2-0 and 40-0 in the second set - so I was almost two breaks down - that was a key moment in the set. And if you give Ayumi a little chance, she will take it. So I'm happy how I got out of that one and eventually won today in straight sets.
"Hopefully I can just take the positives from this match and slowly get better throughout the week. It's just the first tournament of the summer hardcourt season, but we have a long way to go!"
The only other seed in action, No.8 seed Magdalena Rybarikova, wasn't so fortunate, falling to Madison Keys in the night match by the same scoreline, 62 62. It was somewhat the same pattern as well - Rybarikova broke early in each set, going up 2-0 in the first set and holding points for 2-0 in the second set, but Keys came storming back each time and wrapped up the victory in just 65 minutes.
Two college tennis standouts from Stanford were also in action on Day 1, though they had mixed results too, with Mallory Burdette losing a tight one to former French Open champion Francesca Schiavone, 75 63, but Nicole Gibbs - a wildcard into the draw - taking out Kiki Bertens, 64 61.
Vandeweghe, who qualified by beating Sharon Fichman, 64 63, has some very happy memories from this tournament - last year she made it all the way to her first WTA final here as a lucky loser into the main draw, eventually finishing runner-up to WTA legend Serena Williams after a tight two-setter.
"It's always nice to come back to a place you enjoy playing at - and I always enjoy playing here in California and in the US," Vandeweghe said. "From the courts to the California heat that I'm used to, this is a tournament I really enjoy. I'm really happy I could make it through the qualifying."