The 26-year-old from Kazakhstan, ranked just 135th in the world, beat fifth-seeded Maria Sharapova 6-3, 7-5 on Thursday at the Rogers Cup as two recent Grand Slam champions also lost.
Voskoboeva, who cruised past Marion Bartoli and Flavia Pennetta to earn her date with Sharapova, beat the Russian veteran with lovely drop shots and had the 6-foot-2 Sharapova chasing balls back to the baseline all game long.
"Every game was so tight, you know. It's like it was deuce, advantage, deuce, advantage,'' Voskoboeva said. "So I just knew that I have to be focused and to continue playing... anything could happen.''
Sharapova became the latest casualty of a draw which has lost 11 of the 16 seeds.
"It makes for an exciting story because at the end of the day it doesn't matter what you're ranked or seeded, the reason we go out and play the matches is to know who's going to be the winner on that day,'' said Sharapova, who has 23 career singles titles - Voskoboeva reached her first career semifinal earlier this year.
"Whether you're No. 1 in the world or you're facing someone that's 100 or so, you still have to go out and win. That's what the sport is all about,'' Sharapova added.
Hours earlier, No. 6 seed Li Na was knocked out of the third round with a 6-2, 6-4 loss to No. 10 seed Samantha Stosur of Australia. Li became the first Asian player to win a Grand Slam singles title two months ago at the French Open.
Serena Williams advanced with a 4-6, 6-3, 6-3 win over Chinese qualifier Zheng Jie. Williams showed off her powerful serve, firing 10 aces.
After a tough first set, Williams took the momentum in the second and dictated play from there.
"I decided that I had to like really relax and really calm down and just, you know, try to start playing better,'' Williams said. "She was really just playing well from the whole match. I just tried to do better.''
Williams will face Lucie Safarova next. The Czech native downed eighth seed Francesca Schiavone of Italy 6-3, 6-3 to advance.
Also, No. 11 Andrea Petkovic of Germany also reached the quarterfinals after cruising by seventh-seeded Petra Kvitova of the Czech Republic 6-1, 6-2. Kvitova won Wimbledon last month to claim her first Grand Slam title.
No. 3 seed Vera Zvonareva of Russia was also beaten, 6-4, 7-6 (4) by Agnieszka Radwanska of Poland.
Voskoboeva and Sharapova battled point for point before the Kazakh player nearly lost her composure, allowing Sharapova an opening in the match that lasted 1 hour, 47 minutes.
The 24-year-old Sharapova was facing match point three times but Voskoboeva double-faulted twice and then fired a shot into the net. Voskoboeva hurled her racket to the ground in frustration a couple of times pulling herself together, sealing the victory when Sharapova's shot fell short.
"Obviously she played really well this tournament and you could tell today that she came to the match with a lot of confidence, swinging away and going for serve and her shots,'' Sharapova said. "If she could consistently play like that, she wouldn't be ranked where she is today. She showed that she can play really great tennis.''
No. 4 seed Victoria Azarenka of Belarus booked her spot in the quarter-finals with a 6-1, 6-2 win over Maria Jose Martinez Sanchez of Spain, while Roberta Vinci of Italy defeated Ana Ivanovic of Serbia, 7-6 (3), 6-2.
Serena Williams, the tournament's remaining big star, faced Jie Zheng of China in the late match.
Thursday's loss of star power came a day after No. 1 Caroline Wozniacki was ousted. Also gone: second-seeded Kim Clijsters, who withdrew earlier this week due to an abdominal injury, and former world No. 1 Jelena Jankovic and No. 9 seed Marion Bartoli, who were dispatched in their opening matches.
Li was playing her first game of the week after being handed a bye through the first round and then a walkover victory when compatriot Shuai Peng withdrew with an injury.
The rust was obvious as she fired numerous returns long on another blustery day at Rexall Centre, eventually bowing out when she smacked a forehand into the net.
The match was held during a power outage at the venue - a Toronto Hydro problem, according to Rogers Cup organizers. The main scoreboard stayed dark for just over an hour, while the corner scoreboard periodically worked, powered by a backup generator.
The lack of match play was obvious for Li, who fired numerous returns long on another blustery day at the Rexall Centre. She bowed out when she hit a forehand into the net.
"It's always tough after a break to come back for the first match, because I had six or seven weeks that I didn't play a tournament,'' Li said. "At the beginning of the match I didn't even know what I should do on the court, not like during the clay-court season.''
It proved to be another windy day on the court, swirling around the stadium at 15 mph. It caused problems with the serving toss and made for a chilly match for spectators.