Melbourne, Australia (SportsNetwork.com) - Serena Williams' quest for an 18th career singles Grand Slam title will have to wait a few more months. The world No. 1 will have to start a new winning streak, too.
Serbian Ana Ivanovic stunned Williams, 4-6, 6-3, 6-3 Sunday to reach the quarterfinals of the Australian Open.
The 14th-seeded Ivanovic ended Williams' 25-match winning streak and dealt the American a shocking fourth-round exit.
Ivanovic, who was the runner-up in Melbourne in 2008 and was the French Open champion later that year, took advantage of 31 unforced errors from Williams.
"It's not easy playing such a champion and I've never beaten her before," Ivanovic said during an on-court postmatch interview.
Before this match, Ivanovic had lost all four previous matchups against Williams. In fact, she hadn't taken a set against her, but that all changed at Rod Laver Arena.
A five-time Australian Open champion, Williams lost for just the second time in 53 matches at the event when winning the first set. Her only previous defeat came last year in the quarterfinals to Sloane Stephens.
Williams fought back from a break down to win the first set, but in the second Ivanovic broke for a 3-2 edge. She finally finished off the set on her third chance, a forehand winner.
"I had some confidence coming into today's match," Ivanovic said. "I really did certain things extremely well, you know, and I kept her under pressure I felt throughout the whole match."
The errors then started to pile up for Williams, as she lost the first three games of the final set. Williams held serve to get within 5-3 and saved one match point when Ivanovic sent a forehand into the net. The encounter finally ended when Williams sent a return long.
"I wasn't the best, but it was all Ana's day. I thought she played really well," said Williams, who fired 13 aces. "I made a tremendous amount of errors. I'm just not used to missing those shots."
Williams has been dealing with a back injury, which she termed as not "life threatening" but said she was unable to get additional days off due to the schedule of playing essentially every other day as a Grand Slam tournament requires. She even contemplated not taking the court Sunday due to the injury, but gave Ivanovic all the credit for winning.
"I think it's almost good I lost today because now I know what I have to work on," added Williams, who pulled out of the doubles competition with her sister, Venus, before their first-round match. That was because Venus had a lower left leg injury.
Ivanovic will next face 30th-seeded Canadian Eugenie Bouchard, who fought back to beat Australian Casey Dellacqua 6-7 (5-7), 6-2, 6-0.
Only three players have won 18 Grand Slam singles titles in the Open Era - Chris Evert (18), Martina Navratilova (18) and Steffi Graf (22). Serena's next chance at No. 18 will come at the French Open at the end of May.
Fourth seed Li Na of China bested 22nd-seeded Russian Ekaterina Makarova, 6-2, 6-0 in 59 minutes. Li, who lost to Victoria Azarenka in last year's finale in Melbourne, will face Italian Flavia Pennetta in the quarterfinals.
The 28th-seeded Pennetta, who did not play the 2013 Australian Open due to a wrist injury. upended No. 9 seed Angelique Kerber of Germany, 6-1, 4-6, 7-5. This is Pennetta's first quarterfinal appearance at the Aussie Open, but her second straight such showing in a Grand Slam tournament. Pennetta, who fired eight aces, was ousted by Azarenka at the 2013 US Open semifinals.
Since 1999, only one German player has advanced to the Australian Open quarterfinals. That was Andrea Petkovic in 2011.
01/19 06:13:05 ET