Naomi Osaka and Aslan Karatsev are the first through in their respective draws.
Novak Djokovic has beaten Alexander by 3 sets to 1 in their Australian Open quarter final in Melbourne.
The German sixth seed claimed the opening set in a tie-break, before Djokovic rallied to win the second set 6-2.
The reigning champ came back from 3 games to love down to win the third set and won another tie-break in the fourth to win the match.
Djokovic will face Russian qualifier Aslan Karatsev in the semi-finals, after the world number 114 defeated 16th seed Grigor Dimitrov of Belarus in four sets.
Meanwhile in the women's singles Serena Williams was in impressive form, defeating second seed Simona Halep 6-3 6-3.
She'll now face 2019 champion Naomi Osaka in what will be her 40th Grand Slam semi-final.