Stephens powers past Kasatkina to unlock all-American semi-final

The 10th seed, who beat Keys to win the U.S. Open last year, showed impressive patience in some cat-and-mouse exchanges and eventually overpowered the 21-year-old with her bigger game.

Stephens needed 42 minutes to win a first set full of long baseline rallies, but was unstoppable in the second set as Kasatkina's game began to crack.

"This is really exciting for American tennis and I'm really excited to play my really good friend," she said on court.

The 25-year-old had never got beyond the last 16 of the French Open until this year.

"I'm excited to play my good friend in the semi-finals, we've already played in the US Open final and it's really good for American tennis," said Stephens.

The 25-year-old, who was ranked 957 just six weeks before her maiden Grand Slam triumph last year after a series of injury problems, is now into the semi-finals of a major for the third time.

The two players traded breaks early in the match, before Stephens produced some ferocious hitting at the end of a marathon eighth game to give herself the chance to serve for the opening set.

She did just that, with the help of a sumptuous backhand drop shot.

The 14th-seeded Kasatkina kicked off the second set with a solid hold of serve, but Stephens broke in the third game with a venomous cross-court forehand and eased to victory after just an hour and 10 minutes of play.