Earlier Sunday in Arthur Ashe Stadium, Venus dispatched Estonian qualifier Anett Kontaveit 6-2, 6-1 in a fourth-round match that required only 50 minutes.
Thus Serena will have to go through Venus to get one step closer to a rare calendar-year Grand Slam. A win against her sister would leave Serena only two matches away.
"She's playing great," Serena said of Venus in her on-court interview. "I barely had time to get ready for my match because she won so fast. I have to be ready for that.
"At least a Williams will be in the semis, so that's good."
Steffi Graf was the last player, male or female, to achieve the Slam, 27 years ago. Before that, it was Margaret Smith Court (1970) and Maureen Connolly (1953).
This was Serena's 32nd consecutive Grand Slam singles match win, going back to last year's US Open, and her 25th straight in 2015.
"I don't feel any pressure," Serena said. "I wanted that Serena Slam. Winning four in a row at Wimbledon was amazing.
"Now I have a chance to go for it here, five in a row."
In 26 career meetings between the sisters, Serena holds a 15-11 record. Venus won the first encounter at the 1998 Australian Open in straight sets, while Serena took the most recent match, in the fourth round at Wimbledon, also in straight sets.
Serena has won seven of the past eight matches.
In beating the 20-year-old Keys, Serena won in a tidy 68 minutes.
This was a meeting between the player ranked No. 1 for a total of 257 weeks at the end of this fortnight and the one who has been anointed by many as her successor.
Lindsay Davenport, the three-time Grand Slam champion who coaches Keys, said she wasn't worried about her pupil's nerves.
Rather, she was worried that her charge would be too amped up.
Keys played with remarkable composure until the eighth game, when she double-faulted on the final two points to give Serena the first break of the match. Serena held to win the set, coming up with a terrific, deep forehand that led Keys to dump a return in the net.
With Keys serving in the second set at 3-all, a 40-15 lead was squandered. A terrific cross-court backhand from Serena forced an awkward backhand into the net from Keys.
Serena has developed a habit of starting slowly in some of her Grand Slam matches, but on Sunday she was immediately in the match.
"It was really important," she said. "My only chance was to start out fast. I just stayed in there and did the best I could."
Serena modestly credited "a few lucky shots" as the difference.
Keys, meanwhile, completed her best Grand Slam season. She lost to Serena in the semifinals of the Australian Open and reached the third round at Roland Garros and the quarterfinals at Wimbledon -- all career bests, along with her fourth-round finish at Flushing Meadows.
In her on-court interview, Venus was asked about her rivalry with Serena.
"It's been awesome," Venus said. "I'm so proud of Serena, and I think she's proud of me. We inspire each other."
Venus won 51 of 79 points and broke Kontaveit's serve four times. Venus was unbroken.
"I'm so proud of Serena, and I think she's proud of me. We inspire each other." Venus Williams, ahead of her US Open quarterfinal matchup with sister Serena
For Kontaveit, ranked No. 152, it was the seventh match in 10 days and her first appearance in the fourth round of a major.
Venus, the oldest player left in the women's draw at 35, is into the second week of the tournament for the first time in five years. Kontaveit, 19, wasn't born when Venus turned pro in 1994. Venus defeated 18-year-old Belinda Bencic in the third round.
This was a quick match for Venus, which should serve her well against her sister. From 1997 to 2012, Venus had a .500 record or better each year in three-set matches. Then, when the effects of her immune condition, Sjogren's syndrome, set in, she went 6-6 in 2013 and 10-11 in 2014.
This year, despite still not being able to practice for long periods of time, she is 8-3 in three-set matches -- including the first and second rounds of the US Open.
A two-time champion (2001, '02), Venus is playing in her 17th US Open.
The 13th-seeded Makarova made the semifinals at Flushing Meadows a year ago, but Mladenovic, 22, was the more poised player in this one, twice rallying from down a break in the first set. She won with style too, pulling off a between-the-legs shot on the way to taking a point during the second set.
In their previous two meetings, Mladenovic won the first set only for Makarova to beat her in three. There was no repeat this time, with Mladenovic bursting to a 4-0 lead in the final set.
Mladenovic next faces 43rd-ranked Roberta Vinci, who advanced without playing a point when 25th-seeded Eugene Bouchard withdrew from their fourth-round match Sunday after sustaining a concussion when she slipped and fell in the locker room Friday.
Information from The Associated Press was used in this report.