MADRID - World number one Rafael Nadal remained on course to retain his Madrid Masters title with a comfortable 6-1, 6-4 win over Finland's Jarkko Nieminen on Thursday.Nadal had come into the tournament looking to dispel doubts after suffering shock defeats to David Ferrer and Nicolas Almagro in the quarter-finals of the Monte Carlo Masters and the Barcelona Open respectively. However, he looked in fine form as he took nine of the opening 10 games to move 6-1, 3-0 in front. Nieminen then briefly threatened a comeback as he broke back and even had break points at 4-4 in the second set, but Nadal held on before sealing victory when the world number 57 fired long in the next game."I am happy because I think until 3-0 in the second set I played the best I have on clay since I started in Monte Carlo," said Nadal. "It's just a question of finding my rhythm, my level of play and I am doing that little by little. Tomorrow will be another tough match and I need to prepare well for that. "I did what I needed to do today and I just need to find that little bit more consistency which I lost after 3-0 in the second set."Next up for the 13-time Grand Slam champion is a meeting with sixth seed Tomas Berdych after the Czech came through in three sets against Bulgarian 12th seed Grigor Dimitrov 3-6, 6-3, 6-2. Japan's Kei Nishikori continued his fine form on clay after winning in Barcelona two weeks ago as he outlasted eighth seed Milos Raonic in two tie-breaks 7-6 (7/5), 7-6 (7/5). Serena Williams also maintained her quest for a third consecutive Madrid Open title with a 6-2, 6-3 win over Spain's Carla Suarez Navarro.Williams was broken twice in her opening three service games, but Suarez Navarro was unable to consolidate the break and once the American moved ahead 4-2 she saw out the set comfortably. More of the same was to come in the second set as Suarez Navarro held serve just once and, despite her serve being broken twice more, Williams went on to set up a meeting with fifth seed Petra Kvitova in the last eight. "I played Carla on clay I think last year, but in Rome. I feel like she actually played really well today, she made me work for every point and it wasn't an easy win," said Williams. Kvitova was also a winner in straight sets as she overcame Czech compatriot Lucie Safarova 6-4, 6-3. Second seed Li Na had a more difficult morning as she had to come from a set down to beat 16th seed Sloane Stephens 2-6, 6-3, 6-2.The Australian Open champion had no answer to Stephens's power in the first set as she dropped her first set of the week. However, she responded well as one break of serve was enough to take the match into a third set and the Chinese star quickly took control of the decider as she raced into a 4-0 lead before comfortably seeing out the match. "I think she was hitting the ball like pretty heavy. Normally the girls play flatter, but she was playing pretty heavy and it was my first time playing against her so she caused me problems," admitted Li."After the first set I had to tell myself to calm down. After that you can only look forward otherwise you won't find a way back." Next up for Li is a meeting with four-time Grand Slam champion Maria Sharapova or Australian Sam Stosur in the last eight. Polish third seed Agnieszka Radwanska is also safely into the quarter-finals as she thrashed Italian Roberta Vinci 6-1, 6-1.