Roger Federer has faced most top players on tour but never his Australian Open quarter-final opponent Tenny Sandgren, and he is wary of the American as he attempts to set up a potential last-four clash against Novak Djokovic.
The Swiss great swept into the last eight for a record 15th time at Melbourne Park after a first-set wobble against unheralded Hungarian Martin Fucsovics.
He now goes toe-to-toe on Tuesday with the last American in the men's draw who has belied his 100 world ranking by beating eighth seed Matteo Berrettini and 12th seed Fabio Fognini.
The Australian Open is Sandgren's favourite Major and he came from nowhere to make the quarter-finals in 2018, eventually losing to South Korea's Chung Hyeon.
Federer has never played the 28-year-old, and is not underestimating the challenge as he looks to win his first Grand Slam since Melbourne in 2018 and seventh in Australia.
"I wonder why he's not ranked higher, to be honest. Every time I see him play, I feel like he plays very well. He's got a lot of stuff in his game that he's deserving of being higher," said the world number three.
"I didn't follow him that closely. I mean, I remember he played Chung in the quarters the last time I played the semis, to play Chung with his blisters and all that.
"I remember watching that closely because I was going to maybe play the winner."
"It was impressive how he was competing. I'm looking forward to a tough one."
"It seems if I play pretty well, I have a shot."
- 'Huge advantage' -
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"But I feel like maybe you could read his serve better than Isner and Karlovic."