( CNN ) -- The heatwave may have passed at the Australian Open but in its place came shockwaves as women 's title favourite Serena Williams crashed out . The American world No.1 melted under pressure from resurgent Serbian Ana Ivanovic in the fourth round Sunday . Williams , who was chasing a sixth Australian Open crown and an 18th career grand slam , blamed the shock 6-4 3-6 3-6 defeat on an error-strewn display . `` I made a tremendous amount of errors , shots I missed I normally do n't miss , I have n't missed since the Eighties , '' Williams told reporters . `` I 'm just not used to missing those shots . `` I just made way , way , a lot of unforced errors . `` I know for a fact I can play so much better than what I did today , so with that , knowing that , I 'm not disappointed or anything . I just know that I can play ten times better . '' Williams refused to blame a back injury for her defeat although the 32-year-old did reveal that she had considered quitting the opening grand slam of the year before her match against Ivanovic . `` I almost pulled out , '' explained Williams , who last lost a match in August . `` I 'm such a competitor . I mean , I probably should have . `` Maybe I was n't the best physically , but that had nothing to do with it . I do n't want to blame anything . `` I feel like Ana deserves all the credit . I feel she played unbelievable today. `` Victory for Ivanovic is a major boost for the 2008 French Open champion who is trying to get her career back on track after several seasons plagued by big-match nerves , serving troubles and a series of injuries . The 26-year-old told CNN 's Open Court last month that she had effectively pressed the re-set button ahead of the 2014 season , swapping British coach Nigel Sears for new coach and hitting partner Nemanja Kontic . `` I 've been working really hard , '' said Ivanovic , who last reached a grand slam quarterfinal at the 2012 U.S Open . `` I have a new team with me since Wimbledon and it 's a Serbian team for me for the first time . '' The restructure appears to be working as Ivanovic won a warm-up tournament in Auckland -- incidentally beating Venus Williams -- before notching up her first-ever win over Williams ' younger sister Serena . Asked by reporters in Melbourne what the difference was between this match and her previous encounters with Williams , she said : `` Well , I actually believed . I had some confidence coming into today 's match . `` This victory means so much to me . All the hard work , it 's paying off . `` I had very tough fourth round matches in a grand slam , and this was probably the toughest of tough . I just went out there to play . I had nothing to lose . `` To have that victory , you know , it 's amazing . We all know what kind of champion she is . '' Ivanovic goes on to meet Canadian Eugenie Bouchard in the quarterfinals and , after her giant-slaying performance against Williams , the Serbian 14th seed might just feel she has a chance of going all the way . In the men 's draw , Ivanovic 's childhood friend Novak Djovkovic eased into the quarterfinals with a straightforward 6-3 6-0 6-2 win over another close pal Italian Fabio Fognini . Djokovic , who said he had managed to watch two sets of Ivanovic 's match before taking to the court , faces Swiss Stanislas Wawrinka next , who took him to five sets in the fourth round of last year 's Aussie Open . `` I have to be ready to play another 12-10 in the fifth like last year , '' Djokovic said . `` I know that he 's playing the tennis of his life in the last 15 months . `` You can not expect a clear favorite in that match . '' In the same half of the draw , Czech Tomas Berdych also advanced to the quarterfinals where he meets Spanish third seed David Ferrer .
Serena pulled out of the Australian Open after fourth round defeat . Serbian Ana ##ity beats American world No.1 4 - 6 6 - 3 6 - 3 . Williams blamed her loss on a ` ` tremendous series of errors '' In the men 's draw , Novak Djokovic sets up quarterfinal against old foe Stanislas David .