INDIAN WELLS (California) (AFP) Croatian veteran Ivan Ljubicic ousted a weary world number two Novak Djokovic 7-5, 6-3 Wednesday as defending champion Rafael Nadal booked his quarter-final berth at the BNP Paribas Open. While third-seeded Nadal advanced with a 7-5, 3-6, 6-3 triumph over big-serving American John Isner, Djokovic couldnt keep pace with the blistering serves and clean groundstrokes of Ljubicic, a former world number three now ranked 26th in the world. World number four Andy Murray secured his quarter-final slot with ease as Spains Nicolas Almagro retired while trailing with an apparent left ankle injury. Murray led 6-2, 1-0 when Almagro called it quits after receiving treatment on court at the end of the first set. Ljubicic said he was delighted to end a five-match skid against Djokovic, calling some of those defeats brutal. Im really happy to be able to beat him finally, said Ljubicic, whose only prior win against Djokovic was their first meeting in Zagreb in 2006. His most recent defeats to the Serb had come in the first round of last years US Open, and in the quarter-finals at Dubai last month. My serve helped me, especially in difficult moments, said Ljubicic, who gave himself both of his match points in the final game with two of his 13 aces. Djokovic saved the first, but on the second Ljubicic came up with another big serve that the Serbian could only sail back across the baseline. But also two weeks ago in Dubai I was serving unbelievably, Ljubicic said. You cant beat Novak only with a serve, but it does help you a lot if you can get a lot of free points off that shot. Djokovic said he knew what to expect. I dont think any match we play is different, he said. Its more or less based on the service and returns. As much as I get in the rallies, I think I have more chances to win the point. It wasnt the case today. My execution was very bad and he played well when he needed to and he served well when he needed to, said Djokovic, who won his second straight Dubai Open title in February then won both of his matches to help Serbia beat the United States in the first round of the Davis Cup in Belgrade. Djokovic had looked far from sharp since arriving in the California desert, having to save three match points to subdue Germanys Philipp Kohlschreiber in the third round. This is a tournament where I definitely didnt feel comfortable on the court, said Djokovic, the 2008 champion. Its just because I had a very tiring couple of weeks coming from Davis Cup, which was emotionally very exhausting for me. The tournament has now lost its top two mens seeds, after world number one Roger Federer was stunned by Marcos Baghdatis in the third round on Tuesday. That left Spains Nadal as the highest-ranked player to make it into the quarters as he withstood a barrage of 22 aces from the 24-year-old American Isner. Nadal closed it out with a forehand winner into an open court. I am feeling really well, and I am feeling good on court, said Nadal, who is playing his first tournament since retiring injured from the Australian Open. I felt like I played pretty well, Isner said of his first outing against Nadal. I might have run out of gas in the third set, but thats what he does. He makes you work so hard for every point. Nadal next faces Czech Tomas Berdych, a 6-1, 6-3 winner over Serbian Victor Troicki, while Ljubicic will take on Argentinas Juan Monaco, who beat Spains Guillermo Garcia Lopez 3-6, 6-2, 6-1. Baghdatis, who saved three match points en route to his triumph over Federer, couldnt hold off Spains Tommy Robredo, who notched a 7-5, 0-6, 6-4 victory to line up a meeting with seventh-seeded American Andy Roddick, who beat Australias Jurgen Melzer 7-6 (7/5), 6-4. Murray will play sixth-seeded Swede Robin Soderling, who beat Frances Jo-Wilfried Tsonga 6-3, 6-4. Meanwhile womens fifth seed Agnieszka Radwanska of Poland booked a semi-final berth with a 6-4, 6-3 victory over fourth-seeded Russian Elena Dementieva. Radwanska has reached her second semi-final of the season without dropping a set, and denied Dementieva a shot at a third title of 2010 - after triumphs in Sydney and the Paris Indoors. Radwanska will play second-seeded Caroline Wozniacki of Denmark, who outlasted Chinas Zheng Jie 6-4, 4-6, 6-1.