Venus Williams gained plenty of experience in playing out of trouble.
Another former No. 1, Lindsay Davenport, looked comfortable again on the court.
Williams, winner of four Grand Slam events and runner-up to sister Serena in the last three, trailed 1-4, 0-40 in the second set before beating Anca Barna, 6-1, 6-4, Friday to reach the Australian Open's fourth round.
On the men's side, Andre Agassi defeated France's Nicolas Escude, 6-2, 3-6, 6-3, 6-4.
Davenport hit winners even off-balance as she kept Tatiana Panova on the run and won, 6-2, 6-1, in 53 minutes.
Despite problems with wildness, the often dejected-looking Williams needed only 17 more minutes to win. She raised her fist in the air and jumped up and down.
In danger of falling behind 1-5, she saved three break points with a forehand volley, an ace and a deep backhand that Barna could not handle. She double-faulted twice in the final game before overpowering the German, ranked 69th, with a crosscourt backhand.
Williams is seeded second behind her sister, meaning they could only meet in the final. Serena missed last year's Australian Open with a twisted ankle, but then beat Venus in the championship matches at the French and U.S. Opens and Wimbledon.
She next meets Australian Nicole Pratt, who beat No. 23 Paola Suarez of Argentina.
Davenport, winner of three majors including the 2000 Australian, was hanging her head during much of her match Wednesday, when she had to rally from 3-1 in the final set to beat 57th-ranked Iroda Tulyaganova of Uzbekistan.
This time, "I played exactly the way I wanted to play, which was attacking a lot of balls and going for my shots. Today they were falling in," said the 26-year-old American.
Davenport next plays the winner between No. 5 Justine Henin-Hardenne and No. 32 Katarina Srebotnik.
On Thursday, Monica Seles made a surprisingly early exit. Hampered by an ankle injury, Seles lost to 113th-ranked Klara Koukalova of the Czech Republic 6-7 (8-6), 7-5, 6-3 in the second round. It's the first time Seles won't reach at least the quarterfinals of the major she's won four times.
"I was in bad pain; it's an ankle sprain," the sixth-seeded Seles said. "I knew I was in trouble."
The 29-year-old Seles owns nine major titles but hasn't reached a Grand Slam final since the 1998 French Open.
Two young American men hoping for their first real splash at a major reached the third round with victories Wednesday: Andy Roddick and James Blake.
No. 9 Roddick overpowered Romania's Adrian Voinea, 6-2, 6-2, 6-2, and No. 23 Blake beat Argentina's Jose Acasuso, 6-1, 6-4, 6-4. Lleyton Hewitt, a 21-year-old who already has ended the last two seasons at No. 1, brushed aside fellow Australian Todd Larkham, 6-1, 6-0, 6-1.
On the women's side, Kim Clijsters needed just 33 minutes to beat Hungary's Petra Mandula, 6-0, 6-0. After beating both Williams sisters in the WTA Tour Championships last November, she is considered one of the most likely to stop Serena's effort to win a fourth consecutive Grand Slam tournament.
Williams stayed on track for a Serena Slam by beating Belgian Els Callens, 6-4, 6-0.
Seles rolled her left ankle trying to change direction when she was wrong-footed by Koukalova in the third game of their match.
After getting treatment from the trainer while she sat on the court, Seles moved to the changeover chairs to have the ankle strapped and get some painkillers.
She'll have tests Friday.
In the meantime, "I just want to get out of the pain, because I've been in it for the past 21/2 hours," Seles said.