Ash Barty's coach has sent a chilling warning to the world No.1's Australian Open challengers, claiming the title favourite still has at least another gear to find.
In her most ominous start to a grand slam yet, Barty has motored through to the semi-finals without coming close to dropping a set.
The top seed has conceded only 17 games in five matches and is unbeaten all summer, winning a phenomenal 73 of her 74 service games stretching back to her first outing of the summer in Adelaide.
But coach Craig Tyzzer believes Barty has yet to hit her straps - and knows his champion charge will need to against big-hitting American Madison on Thursday night.
Like Barty, Keys will be contesting her second semi-final at Melbourne Park.
"I don't feel like she's played her best tennis here yet, which is great in that sense," Tyzzer said on Wednesday.
"It's still building and she's still got to keep working and being ready for every time she steps out there.
"Obviously Maddie is going to be a huge challenge. Such a powerful athlete and powerful game. Ash has got to be really switched on to be in a contest out there."
Switched on Barty has romped into the last four needing only five hours and four minutes of court time.
But Tyzzer believes it's been Barty's ability to rise to the occasion and find a way to win in challenging matches more than produce her best tennis that has helped the Wimbledon champion progress through the draw.
Now the tennis perfectionist is chasing new levels.
"Last year, we were away seven and a half months. We talked about her best matches. Ash said there was probably four times where she felt like she played her best tennis," Tyzzer said.
"That was seven and a half months of tennis. I think it's her ability to on the days when she's not playing well, still look like she's playing well and still be able to compete.
"Whether or not she brings her margins in or corrects something or tries something different, she's more capable of handling those days when it's not that good.
"I don't think from the outside you can tell. It's probably the same with most of the top players. Their consistency level is huge.
"Even if you watch them, you can't tell a lot of difference."
Barty had been slated to strike defending champion Naomi Osaka in the fourth round, fifth seed Maria Sakkari in the quarter-finals and French Open champion Barbora Krejcikova in the semis.
Instead, the draw has opened up beautifully for the world No.1, with Osaka, Sakkari and Krejcikova all departing prematurely.
"It shows how often the draw pans out like you guys think, hey?" Barty said.
As fate would have it, Barty could end up beating three of the same vanquished opponents from her triumphant 2019 French Open campaign en route to the title match in Melbourne.
Like in Paris three years ago, Barty has toppled Amanda Anisimova and Jessica Pegula already this tournament.
She also beat Keys in the 2019 Roland Garros quarter-finals.
If Barty conquers Keys once more, the dual grand slam champion will face either Polish seventh seed Iga Swiatek or American world No.30 Danielle Collins in Saturday night's final.
Australian Associated Press
