New York - World No 1 Serena Williams stayed on course for the first calendar Grand Slam since 1988, advancing to the US Open semi-finals on Tuesday by defeating older sister Venus Williams 6-2, 1-6, 6-3.
Three-time defending champion Serena reached a Thursday semi-final against Italy's 43rd-ranked Roberta Vinci, who eliminated France's 40th-ranked Kristina Mladenovic 6-3, 5-7, 6-4.
Serena, who holds all four major trophies, moved two wins from completing the first calendar Grand Slam since Steffi Graf 27 years ago and matching Graf's career Open Era record of 22 singles titles, two shy of Australian Margaret Court's all-time mark.
Playing at a high level throughout, the sisters staged a classic at Arthur Ashe Stadium, which was jam-packed with 23 771 fans, including such celebrities as US Presidential hopeful Donald Trump and TV mogul Oprah Winfrey.
"This is a big moment for Venus and I," Serena said. "Very competitive. We were just trying to give each other a great match."
Serena fired 35 winners with 22 unforced errors and 12 aces, the last coming on match point. Venus had 24 winners, 15 unforced errors and eight aces.
Three-time defending champion Serena, the younger sister by two years at 33, improved to 53-2 on the season and took a 16-11 edge in the sibling rivalry, including an 9-5 advantage in Grand Slam meetings and a 3-2 lead in US Open results.
Serena, seeking an Open Era record seventh US Open crown overall, is on a 33-match Grand Slam win streak and a 26-match US Open win streak, her last loss on the Flushing Meadows hardcourts coming to Samantha Stosur in the 2011 final.
Venus, on her deepest US Open run in five years, was seen as one of the big threats to spoil Serena's historic run.
"Losing isn't fun," Venus said. "Probably the most gratifying is I'm still very excited to see Serena have an opportunity to win the four majors. I think that's the best part."
Venus, seeded 23rd, sought her third US Open crown after 2000 and 2001 to add to a Slam trophy haul that also includes five Wimbledon titles, the most recent in 2008.
"She's the toughest player I've ever played in my life and the best person I know," Serena said of Venus.
"It's going against your best friend and the best competitor in women's tennis.
"When I'm playing her I don't think of her as my sister. When you're in the moment you don’t really think about it. For us it's a really great honour."
In the third set, Serena fired a backhand winner to break Venus, fought off a break point to hold for a 3-0 edge and held serve to the end of the 98-minute spectacle.
"Just holding serve in the third set was all I could do," Serena said. "She was blasting so hard. She had so much power. It wasn't easy at all."
The sisters shared a hug at the net at the finish.
"I just said, 'I'm so happy for you,'" Venus said. "I don't remember what else I said after that. Just the moment."
Serena is 4-0 in her career against Vinci, 32, who reached her first Grand Slam semi-final in her 44th Slam start.
"Amazing moment for me. It's unbelievable," Vinci said. "First semi-final in my career. I'm so happy, I don't have words to say."
The Serena-Venus rivalry, born on concrete courts in rough neighbourhoods of 1980s suburban Los Angeles, is the second-most contested at Grand Slam events in the Open Era, their 14 meetings second only to the 22 between Martina Navratilova and Chris Evert.
Serena has won seven of the past eight meetings with Venus, including a win at Wimbledon two months ago.
"It's a big pressure," Serena coach Patrick Mouratoglou said of his pupil facing Venus. "But it's the pressure that helps you lift your level."
The Williamses tested each other emotionally and with some of the best skills in tennis.
Venus saved a break point and held serve in the opening game. Serena broke in the fifth with a forehand winner and the seventh when Venus sent a forehand long, then held at love to claim the first set.
Serena double faulted to surrender her first break, giving Venus a 3-1 lead. Venus held at love, broke again and held when Serena netted a forehand to force a third set, the 11th Serena faced in her calendar Slam run.
Serena could become the oldest Grand Slam champion by capturing her 70th overall career title on Saturday.
Venus was the oldest quarter-finalist at any Slam since Navratilova in 1994 at Wimbledon at age 37. She's the second-oldest quarter-finalist at the US Open in the Open Era (since 1968), about six months younger than Billie Jean King in 1979.
US Open results on Tuesday, the ninth day of the 2015 tournament at Flushing Meadows (x denotes seeded player):
Novak Djokovic (SRB x1) bt Feliciano Lopez (ESP x18) 6-1, 3-6, 6-3, 7-6 (7/2)
Marin Cilic (CRO x9) bt Jo-Wilfried Tsonga (FRA x19) 6-4, 6-4, 3-6, 6-7 (3/7), 6-4
Serena Williams (USA x1) bt Venus Williams (USA x23) 6-2, 1-6, 6-3
Roberta Vinci (ITA) bt Kristina Mladenovic (FRA) 6-3, 5-7, 6-4
Jamie Murray/John Peers (GBR/AUS x8) bt Marcin Matkowski/Nenad Zimonjic (POL/SRB x4) 3-6, 6-3, 7-6 (7/4)
Pierre-Hugues Herbert/Nicolas Mahut (FRA/FRA x12) bt Jean-Julien Roger/Horia Tecau (NED/ROM x3) 7-6 (7/5), 6-4
Martina Hingis/Sania Mirza (SUI/IND x1) bt Yung-Jan Chan/Hao-Ching Chan (TPE x8) 7-6 (7/5), 6-1
Sara Errani/Flavia Pennetta (ITA/ITA x11) bt Lara Arruabarrena/Andreja Klepac (ESP/SLO x 15) 0-6, 7-5, 6-2
Andrea Hlavackova/Lukasz Kubot (CZE/POL) bt Anasatasia Rodionova/Max Mirnyi (AUS/BLR) 6-4, 6-4
Bethanie Mattek-Sands/Sam Querrey (USA/USA) bt Yaroslava Shvedova/Juan Sebastian Chabal (KAZ/COL x6) 3-6, 6-4, 10-6