Defending champion and second seed Roger Federer was the only one of the top seeds to drop a set in his victory over Andreas Seppi, but there were more straightforward assignments for top seed Rafael Nadal, third seed Jo-Wilfried Tsonga and Gael Monfils, the fourth seed.
Federer, seeking his fourth Doha title, made a strong start against Italian Seppi but lost his way in the second set, slipping 1-5 down at one stage before rallying to 5-5. Seppi reeled off two games on the spin to take the set, but Federer recovered in the next set to seal a 6-3 5-7 6-4 triumph.
Next up for Federer is a repeat of last year's semi-final against Tsonga, after the Frenchman eased to a comfortable 6-2 6-1 demolition of Spain's Albert Ramos.
Top-seeded Spaniard Nadal also progressed in straight sets but was made to fight hard for a 6-4 6-4 triumph against seventh-seeded Russian Mikhail Youzhny.
Nadal now faces a tricky encounter with Monfils, who looked in good touch as he saw off the fifth-seeded Viktor Troicki 6-2 6-3.