Big3 (world No. 1-2-3 ) under attack by NextGen in 2019. Stefanos Tsitsipas beat Roger Federer Australian Open + he beat Rafael Nadal at Mutua Madrid Open. Dominic Thiem beat Roger Federer BNP Paribas Open + Mutua Madrid Open, Daniil Medvedev beat Novak Djokovic Rolex Monte-Carlo Masters. What is the next?