The fourth-seeded Indian defeated Juan 21-17 21-13 to make the last-four stage, where she will take on Germany's Juliane Schenk.
Saina was facing Juan after almost three years and the Indian dominated her world number 23 rival just the way she did in 2008.
Saina opened a massive 18-9 lead and it all seemed like a stroll in the park but Juan fought back for a while to reduce the gap to 19-17.
But Saina was unruffled by the fightback and clinched the opening game by notching up three successive points after that.
Saina's domination only increased in the second game and she relied on her trademark smashes to clinch the issue with ease.
The 21-year-old Hyderabadi has a good chance of reaching the finals as well, as she has a good one-on-one record against Schenk.