The most logical reply, and some thought.
1. e2e4  1... e7e5  2. g1f3  2... b8c6  3. f1b5  3... a7a6  4. b5a4  4... g8f6  5. e1g1  5... f6e4 Definitely not a surprise by Mamedyarov. The Open Ruy Lopez was his bread and butter long before he reached Niemann's age. 6. d2d4  6... b7b5  7. a4b3  7... d7d5  8. d4e5  8... c8e6  9. b1d2  9... e4c5  10. d1e2 And Niemann chooses a rare continuation in order to avoid the Azeri's heavy preparation. 10... g7g6  Taken by surprise Mamedyarov chooses \"an interesting sideline,\" at least according to Chessbase.