Eminem has never been hiding his respect to Kendrick Lamar. Some time ago in his interview with The New York Times, he said:
I love Kendrick [Lamar]. I just try to pay attention to what’s out. […] There are certain artists that make me do that just because of the caliber that they rhyme at — it’s like candy to me. Kendrick, the way he puts albums together — front to back, they’re like pieces of art.
His opinion about Lamar’s songs is so high that, according to Ed Sheeran, Em was not completely convinced that Lamar wrote his stuff without any help. Ed Sheeran shared the story he heard from Rick Rubin who worked on The Marshal Mathers LP2. Kendrick Lamar was the only MC who was invited to take an appearance on Eminem’s album but apparently, before getting on the record he had to pass the test:
Kendrick came with all his mates and Eminem said: “I just want you in the studio, just you on your own and then my engineer is gonna come in and then record you doing it, but your mates aren’t allowed in”. Kendrick did it, wrote a sick verse, and then everyone came in to listen to it. Eminem said that he did it to test Kendrick because he thought he had a ghostwriter. He then realised that he didn’t then claimed [Kendrick] was the best.
However, Kendrick did not take offence in such treatment. He described the process of working with Eminem in his interview later:
He brought me out to Detroit, I had a show, and he brought me to the studio to do a hook… I go in, do the hook, and he’s like “I like the hook”. Then I’m like, “Alright, cool, I’m finna dip”, then he says “Think you could do a verse?”… The thing with Em which was crazy to me is he kicks everybody out of the studio… I took it as him kicking everybody out to see if it’s really you writing those raps that you’re writing.
There are no hard feelings between them and after their MMLP2 collaboration they worked together again in 2017 on a tribute to Prodigy, when Hot 97 broadcast a memorial to a late Mobb Deep rapper. Both of them rapped his verses paying respect to an MC who passed away at 42 years old.