The two Americans, who have one clay-court title between them, had never even reached the quarter-finals at Roland Garros before this year, but are through to the last four after convincing victories on Tuesday. Stephens beat the world No 14, Russia’s Daria Kasatkina, 6-3, 6-1, while Keys beat Kazakhstan’s Yulia Putintseva 7-6, 6-4.
Neither American had arrived at Roland Garros in particularly good form – Stephens had won four of her eight matches in European clay-court tournaments and Keys had won two of her four – but they have grown in confidence with their matches here.
Keys joked that she had still found clay confusing as recently as last week. “Obviously I grew up in the States, where we don't really have red clay,” she said. “Even playing on clay, it was green clay, which is much faster and much different. My first real experience on red clay was when I was 16 or 17. It's been a little bit longer for me to get used to it, but I feel like every year I get more comfortable.”
She added: “I obviously lost to Sloane at the US Open, but I feel like on clay it's a little bit of a different match-up.”
Putintseva, the world No 98, was the most unlikely of the eight quarter-finalists. The feisty Kazak, who had reached the same stage of the 2016 tournament, pushed hard in the first set, in which she was a break up, before Keys’ bigger game won the day.
Kasatkina, who had knocked out Caroline Wozniacki in the last round, looked a particular threat to Stephens, but the 21-year-old Russian faded after a tight opening set which was full of long baseline rallies.
“I thought I played pretty solid,” Stephens said afterwards. “I knew that every time you go into a match there are a little bit of nerves that go into it. I knew I had to come out and keep swinging. Sometimes I start well and sometimes a little sluggish. I knew I needed to keep swinging no matter what, even if it was very close, so that's what I did.”
At No 83 in the world Stephens became the lowest-ranked player ever to win the US Open title last September. The 25-year-old Floridian has had a mixed time since. She played six more matches in 2017 after her New York triumph and lost them all, lost again in the first round of this year’s Australian Open and then won the Miami Open.
Keys, the world No 13, has long been considered one of the emerging forces in women’s tennis but in the last three years has added consistency to her undoubted promise. She has played in the second week of nine of her last 11 Grand Slam tournaments.
Stephens, aged 25, and Keys, aged 23, have known each other since they were juniors. Stephens, who describes Keys as “one of my closest friends on tour”, has won both their previous meetings. The world No 10 won 6-4, 6-2 in Miami three years ago and 6-3, 6-0 in an extremely one-sided final at Flushing Meadows last summer.
“When we get on the court, it's time to compete,” Stephens said when asked what it was like to play her friend. “But before that, we are not going to be weird and awkward and make it, like, weird for each other. Everything will be normal.”
Both women made their Grand Slam breakthroughs by reaching the semi-finals of the Australian Open. Stephens lost to Victoria Azarenka in the semi-finals in Melbourne five years ago, having beaten Serena Williams in the quarter-finals, while Keys lost to Serena two years later after beating her sister Venus in the previous round.
Keys and Stephens have both come through serious injury issues in the last year. Keys has had two wrist operations, while Stephens returned to competition at Wimbledon last summer after a 10-month break following foot surgery.
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