"She was totally focused," he said.
Siegemund found that out, too, on Court Philippe Chatrier, as Kvitova rained down six aces — taking her tournament-leading tally to 24 — in a first set blur in which she also won 93 per cent of the points on her first serve and never faced break point.
Kvitova hasn't lost a set in Paris. Kenin, 21, has lost a set in all of her matches bar one, in the third round. But her 6-4, 4-6, 6-0 quarter-final win against fellow American Danielle Collins again showed how winning the title at this year's Australian Open worked wonders for the confidence.
In the other women's semifinal, 54th-ranked Iga Swiatek plays 131st-ranked Nadia Podoroska, the first female qualifier to get to the final four at the French Open.
In the men's draw, the first obvious sign of trouble for Novak Djokovic came when he stepped out on to the court with a large square of beige athletic tape on the back of his neck.
Off to a slow start, Djokovic received massages from a trainer, righted himself and beat Carreño Busta 4-6, 6-2, 6-3, 6-4 to reach the semifinals at Roland Garros for the 10th time.
He will face Stefanos Tsitsipas in Saturday's semifinal, after the Greek fifth seed beat Andrey Rublev 7-5, 6-2, 6-3.
- AP