American youngster Ryan Harrison overcame a stern test to book his place in the Atlanta Open semi-finals.

Harrison made a strong start on first serve against Colombian slugger Santiago Giraldo, stamping his authority on the match with a break midway through the opening set.

The 21-year-old used powerful serving to wrap up the first set in just 26 minutes, but was unable to maintain that high standard under pressure in the second set.

Giraldo struck back with a break early in the second set, much to the visible frustration of Harrison, who broke his racquet before smashing a ball over the compact Metro Stadium.

The Colombian stayed calm to clinch the second set 6-3, setting up a decider for a place in the latter stages.

Both players got visibly tired as the pace slowed early in the third set, with Harrison spurning an early pair of break-point chances after swiftly cancelling out a break from his opponent.

Serving for the match Giraldo seemed to buckle at 0-30 down, but soon recovered with four successive points to force a 6-6 stalemate in the decider and a final set tie-breaker.

Harrison stayed aggressive in the breaker and his efforts were quickly rewarded with a 6-1 advantage.

Giraldo had no answer after saving the first of five match points, succumbing 6-3 3-6 7-6(1), as Harrison followed countrymen John Isner into the final four.