NEW YORK - SERENA Williams kept her bid for a calendar-year Grand Slam on track, powering past older sister Venus 6-2, 1-6, 6-3 on a steamy, star-studded night at the US Open on Tuesday.
Serena moved into the semi-finals, just two wins from history, in what proved the toughest test of her four matches.
The Williams sisters had met 26 times before, but never with so much riding on the result.
A win by Venus would move her a step closer to a first Grand Slam title since 2008 Wimbledon but would scuttle Serena's chances of joining their sport's most exclusive club of calendar-year Grand Slam winners which comprises just three members - Maureen Connolly, Margaret Court and Steffi Graf.
The buzz around Arthur Ashe Stadium was like that of a heavyweight boxing bout and the action on the court was much the same as Serena and Venus put family ties aside and slugged it out like sworn enemies.
"She's the toughest player I've ever played in my life and the best person I know," said Serena in an on-court interview. "So it's going against your best friend and at the same time going against the greatest competitor for me in women's tennis, so it was really difficult today.
"When I'm playing her, I don't think of her as my sister, because she's playing so well, hitting big serves and running a lot of balls down."
The victory set Serena, 33, up for a final-four meeting with unseeded Italian Roberta Vinci.
Vinci beat France's Kristina Mladenovic 6-3, 5-7, 6-4.
While much of the focus for this year's US Open has been on Serena, there was some breathtaking action for those who follow the men's game.
World No. 1 Novak Djokovic was forced to dig deep as he fought off a challenge from 18th-seeded Spaniard Feliciano Lopez 6-1, 3-6, 6-3, 7-6 (7-2) to set up a meeting with defending champion Marin Cilic.
Cilic, the ninth-seeded Croat, was also made to slog, being taken to five sets by Frenchman Jo-Wilfried Tsonga before finally triumphing 6-4, 6-4, 3-6, 6-7 (3-7), 6-4.
Cilic squandered three match points in the fourth set but came up strong in the fifth to stretch his Flushing Meadows win streak to 12 matches.
"It was a big mental fight, especially after losing that fourth set," he said after the four-hour match. "Physically, it was very demanding."