California:Russia's Maria Sharapova aimed to move a step closer to a successful defence of her Pacific Life Open title on Tuesday when she faced compatriot Vera Zvonareva in the fourth round of the event. Top seed Sharapova had won her last three matches against Zvonareva and four of their six career meetings in total. So all signs pointed to a win for Sharapova and it looked as though the 19-year-old would claim a quarter-final place after taking the first set 6-4. The world number one then served for the match leading 5-4 in the second set and won the first point, only for Zvonareva to take the next four to level matters. That sparked a turn around as Zvonareva held her own serve before breaking again to take the set 7-5. Having won the last four games of the second set, number 15 seed Zvonareva took the first four of the decisive third set. Sharapova responded with a break of her own to end the run, but it wasn't enough to avoid a surprise 4-6, 7-5, 6-1 loss. A total of 13 double faults didn't help Sharapova in her bid for victory and she will be replaced at the top of the WTA rankings by Justine Henin, who is not competing at the event in Indian Wells. In the men's draw, there was an intriguing fourth round tie between David Nalbandian and Ivan Ljubicic. Levelling scores Argentine Nalbandian set the pace in the early stages and went on to take the first set 6-2. However, Ljubicic has been in good form this year, winning one title and making three finals appearances. He had also won four of his six career meetings with Nalbandian coming into the match and it was little surprise when the number eight seed forced his way back into proceedings. Ljubicic levelled matters, taking the second set 6-4. Number ten seed Nalbandian failed to regain his hold on the match in the third set and Ljubicic went on to complete a 2-6, 6-4, 6-4 victory. The Croatian had been beaten by Roger Federer in each of the last two years at Indian Wells, but with the world number one knocked out of the event, Ljubicic will be confident of claiming his first Masters Series title at the weekend. Spain's Juan Carlos Ferrero had a tough task on his hands as he looked to clinch a win over his compatriot Rafael Nadal. Following the early exit of Federer, Nadal is the highest remaining seed in the competition, and the 20-year-old certainly lived up to his billing in the fourth round match. Ferrero held his opening service game of the match, but that was as good as it got in a first set that dominant Nadal took 6-1. The second set followed the pattern of the first, with Nadal in complete control on the way to a straight forward 6-1, 6-1, victory. Nadal beat Ferrero for a fifth time in six career meetings to set up a quarter-final encounter with Argentina's Juan Ignacio Chela, who overcame American wild card Michael Russell 6-4, 6-4 on Tuesday.