Williams beat the Latvian in straight sets 6-3, 6-0. She'll face No. 20 Naomi Osaka in the women's singles final on Saturday at the USTA Billie Jean King National Tennis Center.
Williams took 1:06 to dispatch of Sevastova, but fell behind 2-0 in the first set at the start of the match. Then she began charging the net and beating her foe with an array of impressive shots.
"I was just like, I gotta play a little better," Williams told ESPN. "The crowd was so intense and helped me out so much after those first two games. I was like alright these guys want to be out here and see me play, so I want to give them the best that I can."
Williams won 24 of 28 points (86 percent) at the net, compared to 3 of 6 for Sevastova. She also had 31 winners, while Sevastova had 10.
The six-time U.S. Open champion clinched a spot in her 31st Grand Slam final with the semifinal triumph. Osaka beat No. 14 Madison Keys 6-2, 6-4 in a semifinal match following Williams' victory.
Williams missed the tournament in 2017, due to giving birth to daughter Alexis Olympia Ohanian on Sept. 1 of that year. The tennis star is on record saying that she "almost died" during the process. Alexis Olympia was delivered via emergency C-section, after her heart rate dropped dramatically.
The traumatic labor began with a pulmonary embolism -- a condition when arteries in the lungs become blocked by a blood clot. Later, her C-section wound popped open due to intense coughing. When she returned to surgery, doctors found a large hematoma in her abdomen. Williams spent the first six weeks of motherhood in her bed.
All of that was happening while Sloane Stephens was storming through the field in New York, winning her first Grand Slam.
"It's honestly really incredible," Williams said. "A year ago I was literally fighting for my life at the hospital after I had the baby. Everyday that I step onto this court I am so grateful that I have an opportunity to play this sport. No matter what happens in any match, I just feel like I've already won."