NEW YORK — Rarely is finishing a match against the No. 19 player in the world seen as a formality, but such is the nature of No. 1 Serena Williams’s dominance — and the intrigue surrounding her next opponent, her older sister.  Serena Williams was barely halfway through her 6-3, 6-3 win over fellow American Madison Keys when Venus Williams took the podium following her 6-2, 6-1 win over 19-year-old qualifier Anett Kontaveit, but reporters already were asking the 35-year-old about playing Serena for the 27th time as a pro, this time in the U.S. Open quarterfinals.  Venus peppered her answers with “I don’t know what the score is” and “theoretically” and “hoping she gets through this match,” but she still was willing to consider something she has known was a possibility since the draw came out.  “I don’t think anyone wants to be a spoiler,” Venus said of potentially ending her younger sister’s quest for a calendar Grand Slam. “But at the same time, you’re focused on winning your match even though the circumstances are really much different than you.”  This is far from the first time the two have been put in a potentially uncomfortable situation. Thirteen years ago at the U.S. Open, Serena had just overtaken Venus to become world No. 1 for the first time, and in the final, she ended her older sister’s two-year reign as U.S. Open champion in straight sets.  The court in Arthur Ashe Stadium was green then and Serena’s hair was blonde, but the two sisters rationalized the awkward situation the same way they will Tuesday.  “That’s what we always wanted growing up,” Venus said, “Just to be out there on the big stage duking it out when someone named Williams will win.”  And Serena said the faceoffs have gotten more enjoyable with age.  “Nowadays, I would agree. I think it’s more fun than it used to be,” she said. “We really relish the opportunity. We’re both happy to still be involved in getting so far.  And it’s still super intense. She’s doing well, and she wants to win this. So do I.”  [PostEverything: Young black women need to emulate Serena’s will to win]  The consequences of a Serena win are straightforward — it would mean another obstacle hurdled as she chases the first calendar sweep of the four major tournaments since Steffi Graf in 1988.  A win for Venus lacks historic ramifications, but it would extend her deepest run in the tournament since 2010. In 2011, she withdrew from the U.S. Open, citing Sjogren’s syndrome, an autoimmune disease.  She fell out of the top 100 that year for the first time since 1997 but now is in position to move back inside the top 20 with one more win.  Venus won the sisters’ last match on a hard court, 6-7 (2-7), 6-2, 6-3, last year in Montreal, though Serena has taken the other six most recent meetings, including a round-of-16 matchup at Wimbledon this year. Serena holds a lifetime 15-11 advantage.  [Sally Jenkins: Serena Williams wants everything out of life]  “I’m playing, for me, the best player in the tournament, and that’s never easy,” Serena said. “She’s beaten me so many times. . . . She’s a player that knows how to win, knows how to beat me and knows my weaknesses better than anyone.”  No. 9 Marin Cilic, last year’s U.S. Open champion , dropped another set but pulled through to beat No. 27 Jeremy Chardy, 6-3, 2-6, 7-6 (7-2), 6-1, and advance to the quarterfinals.  Afterward, Cilic, who needed five sets against Mikhail Kukushkin in the third round, said having last year’s trophy in his possession has helped him relax this year and that he has been feeling better about his game of late.  “Just feeling really, really good on the court,” Cilic said. “I’m feeling that now even this match I’m hitting the ball better, serving much, much better, and that gives me really good confidence for the upcoming matches.”  Next up for Cilic is No. 19 Jo-Wilfried Tsonga, who defeated fellow Frenchman Benoit Paire in straight sets Sunday.  Young out in mixed doubles  After winning his first six matches across mixed doubles, men’s doubles and men’s singles, Donald Young suffered his first defeat. He and fellow American Taylor Townsend, who made the mixed doubles draw as wild-card entrants, lost their second-round match in a tiebreak to Andrea Hlavackova and Lukasz Kubot, 7-6 (7-3), 1-6, 10-5.  Young is still alive in men’s doubles, in which he and American Michael Russell are slated to face countrymen Steve Johnson and Sam Querrey in Round 3. And in men’s singles, Young is the only unseeded player remaining. Entering the tournament No. 68 in the world, the 26-year-old has come back from a two-set deficit twice and will face No. 5 Stan Wawrinka in the round of 16 on Monday in Arthur Ashe Stadium.  Young beat Wawrinka in a five-set match in the 2011 U.S. Open but lost to him later that year in Shanghai in their most recent match.  Jacob Feldman reports on all kinds of sports for The Post. He previously worked for The Miami Herald, The Boston Globe, and The Charlotte Observer.
The sisters will meet in the quarterfinals as Serena tries to continue her run at a calendar Grand Slam.