MELBOURNE—You never know what you’re going to get from Ana Ivanovic and Sam Stosur on any given day. Big hitters with big nerves, they’ve won a Slam apiece, and they've thrown in their share of meltdowns as well. You also never know what’s going to happen when they face each other. In the seven previous encounters before their third-round match here today, Stosur had won four and Ivanovic three. Most of those matches had been blowouts in one direction or the other.

But while we should have been prepared for anything when they walked into Rod Laver Arena, what we got was still something of a surprise: A high-quality three-set match. Ivanovic, returning serve from inside the baseline and gunning her forehand every chance she got, hit the ball as cleanly as she since she was No. 1 in the world nearly six years ago. While there were the customary nervous eruptions, errant ball tosses, overambitious forehands, and wonky backhands, Ivanovic’s aggression paid off to the tune of 46 winners, and a 6-7 (8), 6-4, 6-2 win over the home favorite.

Yet it was Stosur who got off to the better start, and who looked poised to advance after winning the first set in a chaotic tiebreaker. Sam broke serve with a hard-hit forehand pass at 3-3, then held at love with a gruff “Come on!” for 5-3. Despite being broken after having three set points at 5-4, Stosur returned the favor in the tiebreaker when she saved three set points of her own. At 7-7, rain suddenly poured into Laver Arena and forced both players off court. When they returned, Stosur quickly brought the crowd to its feet by finishing the set with a forehand winner.

Most observers would have expected a disappointed Ivanovic to grow more frustrated after that. Instead, Ana stepped back inside the baseline and began timing her returns beautifully—she knocked off two winners to break at 2-2 and held out from there. The expected hiccup came at 5-4, when she netted a nervous backhand on her first set point, but she came back with a service winner on her second one to send the match to a decider.

From there, Ivanovic, the ancient forgotten confidence suddenly flowing again, was even better. She broke to open the third, held at love for 2-0, saved a break point with another solid forehand at 3-2, and closed the match at 5-2 with a wide serve and a final easy winner.

Ivanovic wasn’t perfect; she made 40 unforced errors. But her willingness to make those errors for the greater, aggressive good was what spelled the difference tonight. It was like old times, a return to the days when we did know what we were going to get from Ana Ivanovic, and we were pretty sure it was going to be good.

In her next match, against Serena Williams, it’s going to have to be even better.