Anisimova grinds down Siniakova in 'crazy' Australian Open clash
The dangerous American, who burst to prominence last year by making the finals of Wimbledon and the US Open, battled past the Czech player 6-1, 6-4 on Margaret Court Arena.
While Anisimova cruised through the first set in 35 minutes she met far stiffer resistance in the second from Siniakova, the reigning Australian Open doubles champion and 10-time Slam winner.
"That was quite a tough match. It was getting a little crazy in the second set," said the 24-year-old, who won titles in Doha and Beijing last year.
"I mean, playing against Katerina, she's such a legend. Just hearing how many Grand Slams she's won every time in doubles is just insane.
"Obviously she was really giving me a run for my money there. But it was an incredible match, and it felt so great to have such a competitive battle out there."
Anisimova is bidding to go beyond the last 16 at the Australian Open for the first time, with her prospects looking good on current form.
She faces 68th-ranked Peyton Stearns next after her fellow American eased past Croatia's Petra Marcinko 6-2, 7-5.
Siniakova failed to pose a threat in the opening set after being broken in game three.
The world number four burst out the blocks again with another early break in the second set and was largely untroubled until a marathon sixth game.
It went to eight deuces before a netted backhand allowed Siniakova to level the set at 3-3 before another ding-dong battle in the next game.
That too went to eight deuces with Siniakova again coming out on top.
But the Czech had used up her reserves and ran out of fight as Anisimova moved up a gear to win the final two games and the match.
Anisimova reached her first major final at Wimbledon in 2025 only to be thrashed 6-0, 6-0 by Iga Swiatek, then made it back-to-back Grand Slam deciders by repeating the feat at the US Open.
She again lost but pushed world number one Aryna Sabalenka hard in a 6-3, 7-6 (7/3) defeat.