Defending champion Damir Dzumhur and Denis Shapovalov are also through.
The Russian ends his career one victory short of 500.
The three-time grand slam champion faces a stern test against Karen Khachanov in the second round.
Pegula goes for her first career title; Parmentier her fourth.
The 19-year-old needed only 56 minutes to oust Monica Puig.
The young American meets Monica Puig on Friday.
The No. 2 seed lost just two games to Olga Govortsova.
The two will meet for a 19th time on Sunday.
Three years after nearly quitting the sport, Del Potro will play for a second US Open title.
The defending champion was forced to retire down two sets to none to Juan Martin Del Potro.
With wins in the last two majors, Bryan surpasses John Newcombe.
The 20-year-old has become the first Japanese player to reach a grand slam final on the WTA Tour.
The 13-time grand slam champion fought off various lulls to defeat Roger Federer's vanquisher in straight sets.
After being overcome with nerves in the 2017 final in New York, the American said she is better equipped to handle the pressure.
Last year's runner-up put in a measured performance to reach another final four in New York.
Each player hit several dozen winners over just shy of five hours.
Sharapova hit more than twice as many unforced errors as winners.
The Belgian first struggled with the injury in Cincinnati.
The straight-sets win followed a challenging five setter two days ago.
The two power players could meet in the semifinals late in the week.