Williams, bidding for her first title at the Cincinnati, Ohio hardcourt tournament, romped to a 6-1 6-3 victory over her Serbian opponent.
Williams needed just 25 minutes to claim the first set against Jankovic. She fell behind 2-1 in the second with an early break, but won five of the next six games, closing out the match with back-to-back aces.
"She's beaten me several times, so I knew today I just had to be focused and do the best I could," Williams told reporters after claiming her 10th win in 14 matches against Jankovic.
"It didn't seem fast," Williams said. "I was just trying my best. And she played some great points. I was just trying to not make many errors, stay focused and do the best that I could."
The top seed wound up with as many winners as unforced errors at 22-22 while the eighth-eeded Jankovic finished made five winners and 24 unforced errors.
Williams, unsuccessful in five previous attempts at Cincinnati, will face Dane Caroline Wozniacki on Saturday for a chance to play for Sunday's title.
The 12th-seeded Wozniacki upended fourth seed Agnieszka Radwanska of Poland 6-4 7-6(5).
Sharapova, seeded fifth, outlasted Romanian Halep 3-6 6-4 6-4 and Ivanovic beat Ukrainian Elina Svitolina 6-2 6-3.
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