Vesnina had clung on grimly as the 18-minute sixth game dragged through an incredible 12 deuces but looked helpless as her rival flicked the switch and stormed home by winning the last four games.
"It was intense. You could run a few miles in that time, or kilometres," laughed the seven-time Grand Slam champion.
As she has shown so many times over the past two decades, Williams fires up best when things aren't going her way. Her refusal to wallow following a bad period of play or after a mistimed shot sees her regularly bounce back with a vengeance.
"Sometimes it is hard to keep the focus when you are winning but whenever my opponent starts to play better I feel it raises my game.
"It was definitely a momentum change and she had the momentum but [I] got a few serves in [and] had a little luck and next thing I know it was 6-4. But that was a crazy game."
The result keeps alive the hopes of fans and tournament organisers for the dream final between Williams and top seed Caroline Wozniacki, after the Dane beat German Julia Goerges in straight sets. Williams will now face compatriot Lauren Davis, who saw off Poland's Urszula Radwanska in the last quarterfinal.
Despite taking her time to find her groove, Williams lifted her game to a new level against her toughest opponent yet.
A heavily strapped right shoulder had many predicting a less than one hundred per cent Vesnina would struggle in the face of Williams' renowned power and for the first set that looked to be the case.
The 2009 finalist had little response to the wide-ranging shots coming from deep and close to the net and no one anticipated the fight she would muster in the second set.
The crowd even got behind the Russian, as she looked capable of achieving the unthinkable, before Williams restored order with a storming finish.
Although the American acknowledges her game lacks perfection, she is hitting the ball sweetly, possesses the confidence to know she can up the ante at the drop of a hat, and looks very comfortable playing at Stanley Street.
It's going to take someone special to halt her progress. Whether that someone is Wozniacki remains to be seen but the tenacious Davis stands in the way first. The 21-year-old showed tremendous spirit to come back from a first set belting to trump Radwanska 2-6 6-2 6-1 in just under two hours.
Despite entering her sixth match of the tournament, the Polish qualifier showed few signs of fatigue as she controlled the opening set. The tide turned early in the second however, as Davis began hitting strongly in the rallies and capitalising on errors to wear her opponent down.
Davis managed a crucial break early in the third set and from there went unchallenged as the week's intense workload took its inevitable toll on Radwanska.
