World number one Alcaraz was superlative in his 6-1 6-3 victory over Grigor Dimitrov and will next play Danish teenager Holger Rune, who continued his brilliant run by beating Andrey Rublev 6-4 7-5.
Young Italian Lorenzo Musetti claimed the best victory of his career by taking out US Open finalist Casper Ruud 4-6 6-4 6-4 while Frances Tiafoe defeated Alex De Minaur 6-3 7-6 (5).
There was a 15th victory in succession for Felix Auger-Aliassime, who is bidding to win a fourth straight title.
The Canadian's 6-1 6-3 success over Gilles Simon brought the French veteran's career to a close.
Simon, 37, was given an emotional send-off during an on-court ceremony after a career that earned him 14 titles and took him as high as world number six in 2009.
Musetti will next play Novak Djokovic after the number six seed came through in straight sets, 6-4 6-1 against Russia's Karen Khachanov.
The victory extended Djokovic's winning streak in tour-level matches to 11 as the Serbian chases a seventh title at the indoor hard-court event.