Lord Gulbis in 5. I feel that the Lord's got a god-mode lurking inside of him that cannot possibly be matched. As long as he can keep his forehand under control, he will be a threat till the end. The man was going toe-to-toe with Zverev from the baseline, and didn't blink (apart from the 3rd set). His drop shot will expose Nishikori's lack of ability at the net, just as it did Zverev.
Not even trolling... Lord Gulbis
possibly presents the biggest threat to Federer on the bottom half of the draw, alongside Nadal (big question mark as well) -- simply because of the lack of certainty surrounding him, and everyone knows his potential. He can be playing just as tight of sets with someone ranked 100+ as anyone in the top 10.