Meanwhile, Kelly Johnson, who had ranked fourth among NL second baseman in the last two ballot updates, has fallen out of the top five.
Upton made his first All-Star appearance in last year's game in St. Louis, while Johnson has never participated in a Midsummer Classic.
This year's game is July 13 at Angel Stadium.
Fans can cast their votes for starters up to 25 times at MLB.com and all 30 club sites using the 2010 All-Star Game MLB.com Ballot Sponsored by Sprint until July 1 at 8:59 p.m. MST. Sprint subscribers can now vote on the go with select Sprint devices. They can learn more about how to vote on their smart phones at MLB.com/Sprint.
Starting rosters will be announced during the 2010 All-Star Game Selection Show on TBS on July 4. Baseball fans around the world will then be able to select the final player on each team via the 2010 All-Star Game Final Vote Sponsored by Sprint.
And the voting doesn't end there. Fans will have the opportunity to participate in the official voting for the Ted Williams Most Valuable Player Award presented by Chevrolet at the Midsummer Classic via the 2010 All-Star Game MVP Vote Sponsored by Sprint.
The All-Star Game will be televised nationally by FOX and around the world by Major League Baseball International. ESPN Radio will provide exclusive national radio play-by-play, while MLB.com will offer extensive online coverage.
Soon, D-backs fans will have to turn their attention strictly to the online voting as the in-stadium balloting ends at Chase Field at the conclusion of the current homestand.
Upton, who made his Major League debut in 2007 at the age of 19, signed a six-year, $51.25 million contract extension with the D-backs this spring.
"We view him as a core player and as we view the next six years the kind of guy we want to build around," D-backs GM Josh Byrnes said at the time.
Last year, Upton was selected as a reserve on the NL team and 0-for-2. He pinch-hit and later stayed in the game at left field, a position he had never played before, as the NL fell, 4-3.