Tomljanovic -- who knocked out tennis icon Williams in the third round on Friday -- advanced to the last eight with a 7-6 (10/8), 6-1 victory over Samsonova.
The 29-year-old world number 46 will play either Tunisia's Ons Jabeur or Russia's Veronika Kudermetova in the quarters.
"The fact that I won, I'm still a little speechless," Tomljanovic said after her win.
Tomljanovic was set on the path to victory after a dramatic opening set which lasted 1hr 22mins. She saved seven set points in an epic near 20-minute 10th game to hold serve at 4-5 down.
She then saved another in the tie-break before clinching the set.
"I don't know that it's 20 minutes ... If I knew it was that long I would probably panic a little bit," Tomljanovic said of the marathon 10th game.
"I think it was just about staying in the moment. I just try to stay as calm as possible because inside I'm dying."
After the marathon the first set, the second set was one more-sided with Tomljanovic wasting no time in wrapping up the victory.
(For more sports news and updates, follow Ahram Online Sports on Twitter at @AO_Sports and on Facebook at AhramOnlineSports.)