Ivanovic hadn't taken a set from Williams in their first four meetings and dropped the first set Sunday before fighting back for a 4-6, 6-3, 6-3 win in a fourth-round match. Ivanovic took 41 percent of the points when Williams served, a very impressive number considering Williams won 71 percent of those points over her first three matches.
Ivanovic was 4-of-14 in break-point chances, another uncharacteristic statistic for Williams who'd see just seven break points (losing one) in the first three rounds.
Williams was able to fire off 13 aces but also had four double faults and 31 unforced errors against 22 winners. She was obviously slowed by a back problem and said she considered not playing Sunday but she didn't use the injury as an excuse.
"But it's OK," Williams said after the match when asked about her back. "It wasn't the best but it was all Ana today. I thought she played very well."
It was the first loss for Williams since Victoria Azarenka beat her in the finals at Cincinnati last August. She fell to 51-2 in Australian Open matches in which she won the first set. The other such loss came against Sloane Stephens in the quarterfinals last year.
Fourth-seeded Li Na routed No. 22 Ekaterina Makarova 6-2, 6-0 in 59 minutes, allowing Makarova a mere four winners. Li won nearly 70 percent of the points on serve and saved all four of the break points she faced.
She is the only player seeded higher than 14th -- Ivanovic -- remaining in the top half of the draw as No. 28-Flavia Pennetta ousted ninth-ranked Angelique Kerber 6-1, 4-6, 7-5 and 30th-seeded Eugenie Bouchard rallied for a 6-7 (5-7), 6-2, 6-0 win over local favorite Casey Dellacqua, who was unseeded.
Pennetta had eight breaks, including three in the final set, and she won 54 percent for the points off Kerber's serve. Bouchard won three-quarters of the points over the last two sets in her win.
Li draws Pennetta in the quarterfinals. They have split their two previous meetings, although they haven't played each other in four years. Bouchard beat Ivanovic in straight sets last year at Wimbledon the only time they played.