Nadal was aiming to extend his record claycourt winning streak to 57 but, having lost all three of his previous clashes against Hewitt, the Spaniard knew he was in for a bumpy ride.
A five-hour outing in the previous round also did Nadal no favours, especially against a player who has a reputation of tirelessly chasing down every ball.
''I was a bit tired after playing a long match (on Saturday)...at the start I didn't have my sparkle,'' a relieved Nadal said at courtside as sweat dripped off his face.
The tone of the match was set in the opening 16 minutes when the first three games went against serve, with Nadal bullying his way to a 2-1 advantage.
Both players fought tooth and nail for every point.
At times Hewitt adopted an attacking approach to put Nadal off balance but the 20-year-old rose to the challenge and backed up his bludgeoning groundstrokes with angled sliced winners.
Despite the one-sided score in the first set, it took Nadal 42 minutes to edge ahead 6-2 when a sliding Hewitt slapped a backhand into the net.
Hewitt refused to cave in.
Aiming to reach the quarter-finals for the third time, the former world number one unleashed his full repertoire of shots to draw level. He secured the second set by breaking his opponent to love when Nadal misfired a forehand.
STONY GLARE The Australian continued to trouble Nadal over the next two sets by retrieving the ball from seemingly impossible angles and was helped by numerous netcords going his way.
One in particular earned Hewitt a stony glare from Nadal, who was not amused when the net foiled his attempt to break the Australian in the fifth game of the third set.
Deadlocked at 4-4 in the third, Nadal conjured a spinning dropshot at break point which proved to be the decisive moment of the three-hour 17-minute battle.
Despite Hewitt's best efforts, Nadal gathered momentum.
An erring lob from Hewitt gave Nadal the third set and he sealed victory when the 14th seed sailed a backhand over the baseline.
''I felt like I was hitting the ball well,'' said Hewitt.
''(But at) the start of the match, he came out smoking it and hitting a lot of heavy balls...I couldn't really dictate.
''When I got my half chance in the second set, I took it.
''But late in the third set and then the whole fourth set, my serve did go off...and that just made it a lot harder to put pressure on him.'' Nadal will play Serbian Novak Djokovic for a place in the last four.
Reuters SK VP0050