Serena Williams has labelled sister Venus Williams the "greatest competitor" in the game after beating her 6-2 1-6 6-3 in the quarter-finals of the US Open.
The world number one came out on top on an emotional night in New York as she continued her bid to complete a clean sweep of the Grand Slams, but she was quick to pay tribute to Venus when reflecting on the win.
"It's a really great moment," the 33-year-old told ESPN. "She's the toughest player I ever played in my life, and the best person I know. So it's going against your best friend and going against, for me, the greatest competitor in women's tennis.
"This was a big moment for Venus and I. It's a Grand Slam, so we both want to do the best that we can.
"It wasn't really easy today at all."
Serena will take on Roberta Vinci in the semi-finals.