Ferrer, who reached the finals at the French Open and the quarterfinal at Wimbledon, came into the match ranked fourth in the world. He was seeded third in Cincinnati's ATP Tour event.
Tursunov, ranked 44th, worked his way through the qualifying tournament just to reach the main draw and has now won six straight matches.
Tursunov also defeated Ferrer earlier in the year, at the Barcelona Open Banc Sabadell, a clay court tournament, in April in their only previous meeting.
The 30-year-old Russian needed just over an hour to dispense with Ferrer in a dominating performance. Tursunov won 78 percent of his first serves, including six aces.
Argentina's Juan Martin Del Porto, seeded seventh, beat Feliciano Lopez, 6-4, 1-6, 6-4, to advance tpo the quarterfinal, where he will meet Tursunov on Friday.
Tursunov defeated American James Blake, 6-4, 6-4, in the second round on Wednesday.
Three Stanford products were scheduled to play in doubles matches Thursday night. Bob Bryan and Mike Bryan, the world's top-ranked doubles team, were scheduled to meet France's Jeremy Chardy and Richard Gasquet while Scott Lipsky, with teammate Santiago Gonzalez advanced in the doubles tournament, beating sixth-seeded Robert Lindstedt (Sweden) and Daniel Nestor (Canada), 7-6, 6-4, on Wednesday.
In other news, Stanford sophomore Nolan Paige and Menlo School grad Andrew Ball each has a chance to qualify for a wild card at the U.S. Open.
They will participate at the U.S. Open National Playoffs, which is run concurrently with the New Haven Open on the campus of Yale University beginning Friday.
Ball, who will be attending Harvard in the fall, drew a first-round bye and will meet the winner of a match between Paige and Tony Larson in the second round.
Ball qualified for the tournament by winning an event in Salinas. Paige made it to the playoffs with a tournament win at Yale.
"Nolan had a good fall but then he lost some close matches and maybe his confidence wavered a little bit," Stanford coach John Whitlinger said. "This summer he has been working hard and I look for him to have a really good year."
Menlo College grad Jimmy Bosco and Palo Alto's B.J. Boyd rejoined their respective teams in the New York-Penn League this weekend with a new title: all-star.
The two athletes were on different sides of the National League affiliates vs. the American League affiliates on Tuesday night at Dodd Memorial Stadium in Norwich, Conn.
Bosco may have had the better game but Boyd played for the winning team.
Bosco, a member of the State College Spikes, the Single-A affiliate of the St. Louis Cardinals, entered the game as a pinch hitter and singled and doubled in his two plate appearances. He played right field. Bosco, named to the all-star team as a reserve, is hitting .282 on the year.
The American All-Star Team scored twice in the bottom of the ninth to earn the 4-3 victory. Boyd, with the Vermont Lake Monsters, an affiliate of the Oakland Athletics, was voted in as a starter for the AL. He went 0 for 2.
Boyd, hitting .320 on the season for the Lake Monsters, started in the No. 3 spot in the batting order and played left field.
Men's water polo
Stanford junior Bret Bonanni scored six goals to help the United States junior national team beat Italy, 19-6, in the FINA junior world championships in Szombathely, Hungary on Thursday.
The Americans meet Italy in Friday's quarterfinal.
Stanford's Alex Bowen and Adam Abdulhemid are also playing for the U.S.
Stanford freshman Casey Danielson came from two holes down with four to play to half her singles match play round with England's Georgia Hall to help Team beat Team Europe, 14 1/2 to 9 1/2 in PING Junior Solheim Cup action at the Inverness Hotel and Golf Club in Englewood, Colo. on Wednesday.
Danielson and Hall led off the singles competition, with Hall taking a quick 1 up lead on the first hole. Hall never trailed, though Danielson squared things on the 16th hole with a birdie to Hall's bogey.
Hall and Danielson each hit par on the 17th hole and each birdied on the 18th hole as the U.S. won the Cup for the third year.
The teams were tied at 6-6 following the first day of action Tuesday. Danielson and Hall were one of three halved matches and the U.S. won seven of the matches.