The Serbian has had a difficult couple of years since winning the French Open in 2008 and reaching the top of the rankings.
She has now fallen to 58 in the world but this week has seen her rediscover some of her old form.
Russian Petrova was the third seed in a row that she knocked out following Viktoria Azarenka (ninth) and Elena Dementieva (fifth).
And Ivanovic said a good part of her renewed form is down to her serve.
"We have been working a lot on my serve and honestly it needed a lot of improving and it is still work in progress," she said.
"I still think I can serve a lot better and I am getting to a point where it is a great feeling and it gives me a lot of confidence.
"(Thursday) at a certain moment I served really well and also (Wednesday) night and so it is good to get good feedback from all of the work that I put in.
"It is an important part of the game and Nadia was serving really, really well and it was great for me to be able to hold my serve because it was very difficult for me to return hers."
She also feels that her improvement had been coming these last few weeks.
"Well honestly, I have been feeling really good on court for a couple of weeks now and I really felt it could happen any week," she added.
"I just needed to get some more momentum and start playing matches because that is what I have been lacking for so long.
"I am so happy that I came here because last week it just was not happening. To have these wins here means a lot."
Next up for Ivanovic is the unseeded Spaniard Maria Martinez-Sanchez who stunned world number two Caroline Wozniacki in the third round before earning her last four spot with a straight sets victory over Czech Lucie Safarova.
But it will be a bit of unknown territory for Ivanovic.
"I don't know much about her actually. We did play Fed Cup against her last year in Spain but she was playing against another player, so I didn't play against her.
"I also saw her play a few matches this week and she seems to be in really good shape and serving really well.
"She's very different from a lot of girls out there today, she has a serve and volley game and uses drop-shots well, so it's going to be tough.
"I just have to keep playing the way I did and I just really enjoy being out there and competing and having the opportunity to play more matches."