Sasuke wasn't that far ahead when they met last time. Maybe he is now, that Naruto decided he wont use Kyuubi anymore, but still.
At the time Naruto had just fought against Orochimaru and got wounded by the Kyuubi chakra to the point that he was covered in his own blood.
That's quite a strain, even for kyuubihealing Naruto.
There was nothing he could do. He was tired to the point he had to sit down and rest! In the middle of Oro's lair!
Then there's the fact that he can't fight Sasuke for real. Not with the intend he'd need to win against a strong enemy.
But imagine this.
Sasuke charges at Naruto to kill him, but hits Sakura because she is doing he standing-in-the-middle-to-stop-them thingy.
That would be the reason for Naruto to be serious.
He'd grew 4 tails in an instant.
And that'd be it for Sasuke. If Oro couldn't do anything apart from running away, why should Sasuke.
He could try to use his little Chidori sword, but if Oro's original Kusanagi can't do the trick, neither can Sasuke's.
One mouth Rasengan or whatever it was and the fight would be over and Sasuke would lie there beaten to a bloody pulp.
And stop insulting Kishi (or Oda for that matter) for things you don't like and/or understand.
Just stop reading the manga, keep it to yourself and leave this place.
And give us, who enjoy this and other manga, a rest
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"