Harper clubbed his seventh homer, giving him a six-way tie for second in the South Atlantic League, as Class A Hagerstown beat Lexington, 8-2.
The 18-year-old outfielder has been on a power surge of late, raising his average to .357 with a two-run go-ahead shot in the third inning. He finished 1-for-3 with a walk and two runs scored, extending his hitting streak to nine games.
Harper broke a 1-1 tie in the third when he launched his shot to right field off Legends starter Juan Minaya, who gave up a double to leadoff man Blake Kelso. It was his second homer in five games and fourth in his last nine for the Suns.
The No. 1 overall pick in the 2010 Draft trails only Greenville's Miles Head, who has nine homers. He also ranks fifth in the league in total bases (59) and on-base percentage (.455).
Trailing in the seventh, 4-2, the Legends' Gabriel Garcia intentionally walked Harper to load the bases. But Suns catcher David Freitas, a 15th-round pick last year, followed with a grand slam to left, his third homer of the year. He finished 3-for-4 with five RBIs.
MLB.com's No. 3 prospect, is batting .357 with 23 RBIs, eight doubles and five steals through his first 25 games in the Minors.
Lefty starter Bobby Hansen (3-1) got the win, holding Lexington to two runs on eight hits and a walk over five frames.