User blog comment:IamJakuhoRaikoben/Chapter 444: Emperor Spriggan, Review./@comment-24546844-20150713134245/@comment-24039250-20150713160845

He probably saw him either during the last arc, on Tenrou, or maybe even through the tellings of him by his father Yury who met Zeref 100 years ago.

I can't wait to see what goes down between the two.
 * Zeref: Hey Makarov.
 * Mararov: Yeah?
 * Zeref: Thank you for taking care of my brother for all these years.
 * Makarov: Oh no probl.....WAIT, YOU HAVE A BROTHER?!?!?...wait...I KNOW YOUR BROTHER?!?
 * Zeref: Yep. It's Natsu.
 * Makarov:.....I should be more surprised...but for some reason I'm not.