There was no build up for King vs Sanji. Sanji was just the next person to arrive to try to rescue Momo. King give zero f's who Sanji is. This wasn't a scenario where his friends were trying to escape and Sanji jumped in front of King to protect everyone. He was even using his RS invisibility to avoid a battle.
It's silly for people think that Sanji, who couldn't even damage P1 with his RS, will beat King, a much more powerful and deadlier opponent. Yes, I do believe the RS is Sanji's powerup but it's not that big of a powerup. When the chapter left then Sanji he was concerned King was going to put a hole in through his stomach because you couldn't escape. King was doing was just flying and pushing against Sanji with his beak, which hasn't showed to be razor sharp.
Flight is not a fighting style, but swordsmanship is. Characters in One piece use flight is used to either travel or to build momentum to produce powerful close range attacks. That includes using it to change the distances between opponents. Take a look at Sanji and his family for example. Doffy can only move to the air if he has clouds he can connect his strings to. Even Luffy was only using his flight ability to travel after Doffy after he was punching him across the island. That same power gave him no advantage over Cracker or Katakuri.