The No.5-seeded Stosur, who accepted a wildcard into Carlsbad after losing her opener in Stanford, came from 3-1 down to win a marathon 57-minute first set then rolled past Varvara Lepchenko, 75 61.
"Overall it was pretty good," Stosur said. "I got off to a little bit of a slow start, losing my first two service games - that isn't the way you want to start matches. But if you start poorly then end well, that's all that matters. I got through that little rough patch at the start and then found my groove a little bit more.
"After I got over that first set I probably loosened up and from there played quite well."
Stosur wasn't originally planning to play here but shifted gears after her early loss in Stanford.
"I had planned just to go to Stanford last week, then take a week off and head back to my training base in Florida," Stosur said. "I was going to practice for a week before heading to Toronto and Cincinnati. But after not doing as well as I wanted to last week, I thought coming here would be another good opportunity to play those matches and try and build on something a little bit. Being so close it was a good idea to come here and play, and obviously I'm very lucky to get that wildcard this week."
How good does the 2011 US Open champion feel to get her first win of the summer hardcourt season?
"A big part of it is just trying to win matches," Stosur said. "No matter how bad or good you play, if you win, you always feel better. Last week was pretty disappointing, so to come here and win today is a nicer feeling, and hopefully I can really build on that and play better and better from here on in."
Stosur was one of three Grand Slam champions in action during the day session, though not all of them won. 2010 French Open champion Francesca Schiavone missed out on a match point in the second set but regrouped to beat Flavia Pennetta in a nail-biter, 76(4) 67(6) 64, while French wildcard Virginie Razzano beat 2004 US Open and 2009 French Open champion Svetlana Kuznetsova, 62 64.
"Mallory was playing so well - she was hitting the ball unbelievable," Jankovic commented. "Credit to her, she played at such a high level. I had to fight hard and at the end I was able to stay strong mentally and physically to finish it off. I'm happy with the win. It was a very difficult match tonight."