ROME (AFP) - Rafael Nadal quickly put behind him what he described as his worst clay performance in memory to streak into the Rome Masters quarter-finals with a 6-4, 6-2 win over Feliciano Lopez on Thursday. Just 24 hours earlier, the world number one cut a frustrated figure following his troubled three-set victory over Italian qualifier Paolo Lorenzi in the second round. He told reporters after the game that he had played badly and could not remember a worse performance on his favourite surface. Some suggested he might have been adversely affected by his loss to recent nemesis Novak Djokovic in the Madrid final on Sunday -- his first clay defeat in 10 outings against the world number two. Nadal insisted he was feeling no pressure and said it would not be possible to play so badly again, before revealing that his poor form was due to a fever. "I had a fever last night and this morning I didn't feel perfect. Before the match I didn't know if I go on court," he said. "I had a practice at 11:30 for just 15 minutes to see how it went and finally I decided to go on court. It worked a little bit better than yesterday. It's something that is impossible to control. It's strange for me, usually I never have this kind virus or fever but it's the second time for me this year. I don't know if it's the tournaments, changing places, having to adapt very fast to different places. The last few months have been hard. I certainly didn't understand very well yesterday (Wednesday) why I felt that slow and without energy. I understand better today. I'm here to fight, I will try to do my best like I always did." Despite conceding that he did not feel on top form, Nadal was more like his old self as he notched his fourth straight sets victory against his countryman on this surface. He began with an early break in the second game as Lopez went long on a sliced backhand. But just as he had failed to hold a break in the first set against Lorenzi, Nadal suffered a break back against his compatriot. And it was a similar failing to the day before, when his forehand was out of sorts, that cost him as he plonked one into the net on break point in the seventh game. However, Lopez played a sloppy game when serving to stay in the set at 4-5, a poor drop-shot into the net bringing up 0-40 and three set points to Nadal, who took the first with a rasping backhand passing shot. Nadal needed only one opportunity in the second set as he hit an inside-out forehand winner to bring up two break points at 3-2 and then Lopez went long to hand his countryman a decisive advantage. A ruthless Nadal did not let Lopez win another game, breaking him again in the eighth game. In fairness, Lopez was far from his best, hitting 30 unforced errors to just 12 by Nadal, including on match point, when he went wide with an easy sliced backhand. There were mixed fortunes for the two Australian women remaining in the draw. Sixth seed Samantha Stosur eased into the quarter-finals as she put an end to Slovenian qualifier Polona Hercog's impressive run, winning 6-3, 6-4. However, Jarmila Gajdosova was soundly thrashed by Chinese fourth seed Li Na, 6-1, 6-2. Twice a former champ here, the former world number one Jelena Jankovic was made to fight to reach the quarters. The fifth seed dropped the second set before overcoming Spanish qualifier Medina Garrigues 6-4, 2-6, 6-2.