The surprise of the men's draw has been Russian qualifier Aslan Karatsev, who will be looking for a semi-final place in his first Grand Slam tournament when he faces Bulgaria's 18th seed Grigor Dimitrov.
The Japanese three-time Grand Slam champion overcame the Taiwan world number 71 unorthodox style 6-2, 6-2 in an all-Asian quarter-final at a crowd-less Rod Laver Arena.
Osaka then lifted her intensity, opening to a 4-0 lead before Boulter unexpectedly peeled off three straight games. But the Japanese star refocused to force a deciding set, which she ran away with.
The 29-year-old Wimbledon champion had an eventful 2020 season in which she ran up a 17-match winning streak.
The start to her 2021 campaign will involve the long journey to Australia in mid-January before two weeks in quarantine ahead of the start of the year's first Grand Slam on Feb. 8.
World number two Halep, who had skipped the US Open due to COVID-19 concerns and opted to stay in Europe and practice on the claycourts ahead of the French Open.
Halep had received a bye into the second round but was far from her best on serve and was broken five times in the match.