"They're not going to go away," Nashville coach Peter Laviolette said. "There's a reason why they sit where they sit in the standings, and we knew that."
Watson put the Predators ahead 3-1 when he scored on their 13th shot on goal of the game at 16:49 of the second period.
Lewis cut the lead to 3-2 at 3:56 of the third. Kings forward Alex Iafallo, a healthy scratch the past two games, helped win the puck on the forecheck, and then passed to Lewis cutting down the middle.
Watson deflected a shot from defenseman Ryan Ellis for his second goal of the game at 5:05 to put the Predators up 4-2.
"He got rewarded tonight, going to the hard areas," Laviolette said of Watson. "He does a lot of the dirty work for us, so it was nice to see him chip in offensively."
Toffoli made it 4-3 at 8:24, but Rinne made five saves the rest of the way to preserve the lead.
"Probably the most exciting thing about this group is they have a way to stick around in hockey games," Kings coach John Stevens said.