MELBOURNE: Top seed Rafa Nadal was rattled by a feisty challenge from Diego Schwartzman but held firm to fend off the diminutive Argentine 6-3 6-7(4) 6-3 6-3 to reach the quarter-finals of the Australian Open on Sunday. In an often breathtaking clash boasting some sumptuous shot-making, the Spaniard was stunned by the hard-hitting Schwartzman who broke him three times before claiming the second set at Rod Laver Arena. But the 16-times grand slam champion responded with typical grit, raising his defensive game to blanket the 24th seed's firepower and claiming the win on the third match point with a rocketed backhand return down the line. The victory ensured Nadal will retain his world number one ranking and set up a meeting with sixth seed and former U.S. Open champion Marin Cilic for a place in the semi-finals.