Please Welcome to the Stage: Every Single Surprise Taylor Swift Guest

Who?! You don't know about Serena Williams, one of the world's top athletes? The woman who holds six Wimbledon titles and has 18 Grand Slam victories under her belt? Go to your room.
Where?! Serena hopped into that epic group of ladies who took the stage during Taylor's London tour.
Why?! She's one of the top athletes in the world, OK?!