Self-belief and willpower played large parts in one of the biggest upsets in women’s tennis in years.

The rest was down to courage, hard work and a bad back.

Ana Ivanovic scored her first ever triumph over world No.1 Serena Williams in a fourth-round Australian Open match that is one of the best she’s ever played.

Ivanovic, the 14th seed, toppled the world’s most dominant player 4-6 6-3 6-3, suggesting she may have relocated the form that carried her to the top of the world rankings six years ago and then mysteriously deserted her.

“This victory means so much to me,” Ivanovic said. “I worked so hard and all the hard work … it’s paying off.”

Ivanovic also held fast to a plan that required her to disregard Williams’ 17 grand slam titles, her record number of Australian Open wins and the weight of her sheer presence across the net.

“I didn’t think much about who I was playing because she can be overwhelming,” she said. “I had to break her spell.”

Ivanovic had lost each of her four previous meetings with Williams, failing to even win a set.

But this time she believed from the start that an upset might be on.

“When we started the match, I really felt I was in it. I felt if I keep doing the right things I have a shot at this.” she said. “Even though I lost that first set … I kept believing and I kept fighting and looking for my moments.”

She found them in a way she hasn’t since she won the French Open in 2008, the same year she reached the final in Melbourne.

But Ivanovic hasn’t been back in the top 10 since then, winning only two WTA titles in the past six years.

“I think all the struggles were for moments like this,” she said. “You work hard and you never know what’s going to happen.”

Williams, who had appeared untouchable in her first three games at this Open, seemed lethargic and discouraged against Ivanovic.

She initially gave all credit to her opponent, but eventually acknowledged she had a back problem, ultimately agreeing to the proposition that she had almost pulled out of the tournament.

“Well, yeah, I probably should have, but I’m such a competitor,” Williams said. “It’s not like I gave her the match, but I almost didn’t play. But I did, and at least I feel good that I tried the best I could.”

Ivanovic joined two-time beaten Australian finalist Li Na in the quarters after the Chinese fourth seed scored a dominant fourth-round win over Russia’s Ekaterina Makarova.

Li took less than an hour to dispose of Makarova 6-2 6-0, setting up a quarter-final against Italy’s Flavia Pennetta.

Li has led in both her finals in Melbourne – against Kim Clijsters in 2011 and Victoria Azarenka last year when severely hampered by an ankle injury.

Pennetta, the 28th seed, overcame more serious opposition from German ninth seed Angelique Kerber before registering one of her best grand slam results, winning 6-1 4-6 7-5.

Pennetta’s run at this year’s championship is her best in 10 appearances at the Australian Open and follows her semi-finals appearance at last year’s US Open.