Juan Martin Del Potro. Pic/AFP

Former US Open champions Marin Cilic and Juan Martin del Potro powered into the fourth round of the Miami Open on Sunday with straight-sets victories. Argentina's fifth-seeded del Potro, chasing his second ATP Masters title in as many weeks, breezed past Japan's Kei Nishikori 6-2, 6-2. Ranked sixth in the world after vanquishing Swiss great Roger Federer in the final at Indian Wells last weekend, del Potro needed just 73 minutes to book a fourth-round meeting with Filip Krajinovic of Serbia, who beat France's Benoit Paire 6-3, 6-3. Second-seeded Cilic also reached the fourth round with a 7-5, 7-6 (7/4) victory over Canadian Vasek Pospisil. Cilic produced six aces and saved all three break points he faced.