An article in the New York Times reported on this incredible feat by Frank van Aken. My comment -- The story is even more amazing than I originally thought. FTA - "Mr. van Aken, who last sang the role 16 months ago at La Scala, also as a late replacement, participated in a quick rehearsal on Friday afternoon with Fabio Luisi, who is conducting many performances in the cycle, and had only a few moments to try out his voice on the main stage, which was scheduled to be used for a lecture." So not only is he stepping into a strange production with no real rehearsal at one of the best opera houses in the world singing a major role, BUT HE HADN'T SUNG THE ROLE IN 16 MONTHS!!
Am adding a comment by Sam McElroy in response to another article about this amazing story -- "I can not imagine the night he must have had last night! He was probably in holiday mode, relaxing, then suddenly... Siegmund at the Met! He may have been tired from traveling, dehydrated, jet-lagged, enjoying a few drinks, in any frame of mine BUT that of singing Siegmund! He would have had to look through the score, maybe a production video (handy having his Sieglinde there with him, of course!) get himself psyched up, sleep - if even possible - be up early to wake the voice up.... What a brave guy! If you have never sung an opera you can have no idea of the scale of what he achieved today! Imagine an athlete running an olympic marathon within 18 hours notice, and no tailored preparation! I hope he managed to negotiate a return to the Met for a full run of something else. It is nothing short of what he deserves! Bravo, Frank van Aken!"