Roger Federer of Spain and Rafael Nadal of Spain have entered the quarter-finals of the Qatar Open .In Doha today Defending champion Federer beat Slovenian qualifier Grega Zemlja 6-2 ,6-3 to set up a meeting with Italian Andreas Seppi. World number two Nadal beat Germany's Denis Gremelmayr, also a qualifier, 6-2 6-2 and will now meet Mikhail Youzhny.French seeds Jo-Wilfried Tsonga and Gael Monfils also made progress. Third seed Tsonga beat Italy's Flavio Cipolla 7-6 , 6-3, while Monfils, seeded fourth, overcame Benjamin Becker 7-5, 4-6, 7-5.Viktor Troicki, seeded 5th, beat Matthias Bachinger 7-6 ,6-1, Youzhny, seed no.7, beat Ivo Karlovic 6-2 6-7 6-3 and Seppi, 8th seed,saw off the challenge of Guillermo Garcia-Lopez 7-5 6-3.A full complement of seeded players in the last eight was spoiled as sixth seed Alex Bogomolov Jr withdrew with an ankle injury and Spaniard Albert Ramos, ranked 66 in the world, went through on a walkover.