Ivanovic was erratic against the resilient, No. 97-ranked Dechy, who saved two match points on her own serve in the 12th game of the third set.
Ivanovic set up three more match points at 0-40 in the 18th game and, after Dechy saved one, the 20-year-old Serb squealed with delight and kissed the net after hitting a forehand winner to end it in 3 hours, 24 minutes.
"It was an amazing match," Ivanovic said. "In the second set I saved some match points and from that point on I just thought it's my second chance."
She said her heart skipped a beat when she won match point.
"Once it went in I just couldn't believe it," she said. "I'm just so, so thrilled."
Ivanovic next plays China's Zheng Jie, a 6-2, 7-5 winner over Britain's Elena Baltacha.
Also advancing were No. 4 Svetlana Kuznetsova, who beat Katerina Bondarenko 6-2, 6-3; No. 8 Anna Chakvetadze, who cruised past Edina Gallovits 6-4, 6-2; and No. 18 Nicole Vaidisova, a quarterfinalist last season, who dropped nine straight games to fall behind a break in the third set before recovering to beat Australia's Samantha Stosur 6-2, 0-6, 6-4.
Ruano Pascual, at 34 the oldest woman left in the draw, looked impressive in the first set while Mauresmo struggled with a stiff knee but was easily swept aside by the Frenchwoman, six years her junior, in the next two.
Although Mauresmo dropped just two more games against the nine-time Grand Slam doubles champion, she said she was still feeling the injury that forced her to withdraw from last week's Eastbourne tournament.
"[I was] really a little bit tight in terms of movement at the beginning, I mean, the whole first set," Mauresmo said.
"I didn't really know how the leg would react, so I was probably a little bit scared and [had] some tension on that for most part of the first set," she said.
The former world No. 1 knows she will be up against it, facing Williams in the next round having lost both their previous meetings here in the 2002 and 2004 semifinals.
"Tough, definitely," she said of the draw. "I haven't seen her play yet here but I basically know her game pretty well and what she's going to do and what to expect. I hope I'll be ready in a couple of days for this match."
Asked about her chances beyond Friday's clash, the double Grand Slam winner was understandably guarded.
"We'll see. We'll see how physically it goes. If I can get through the next one, we'll talk about it after maybe," she said.
Kuznetsova was relieved to enjoy an easy
afternoon on Centre Court after having been forced to come from a set
behind to sneak past French qualifier Mathilde Johansson in the first round.
"I think I did everything to have it easier," the former
U.S. Open champion said.
"I played much better, and I was just playing a little bit
more like you should play on grass, not on clay court. For me
it's difficult to adjust."
Bondarenko, the world No. 47 and a junior winner here
four years ago, rallied in the second set but was unable to
recover from falling two breaks behind in each set.
Kuznetsova, a three-time Wimbledon quarterfinalist, was
equally disappointed with her service game, which she gave away
twice in an otherwise error-free performance.
"I was really happy with everything except my serve," she
"I could not get my serve in as much as I would like to. I
was maybe wanting to do too much with this, and I wanted to go
more forward, which I think I did pretty well."
Kuznetsova will hope to celebrate her birthday on Friday
with victory over another qualifier, Czech Barbora Zahlavova
The world No. 109 made it to a Grand Slam third round for
the first time when Bondarenko's older sister, the 28th-seeded
Alona, retired injured trailing 3-6, 0-3.
Information from The Associated Press and Reuters was used in this report.