Australian tennis young gun Thanasi Kokkinakis has notched one of the best wins of his career at a Challenger tournament in Germany.

Kokkinakis, who only turned 18 last month, upset seasoned American third seed Michael Russell 6-2 6-1 to reach the quarter-finals in the claycourt event in Neckarcup.

At 36, Russell is twice the age and vastly more experienced but Kokkinakis crushed the American on his favourite surface.

Russell’s crowning glory came on the red dirt at the 2001 French Open when he charged all the way to the last 16 and pushed then-world No.1 and eventual champion Gustavo Kuerten to five sets in the fourth round.

Kokkinakis’s confidence-boosting win over the world No.93 was his first top-100 scalp since beating Dutchman Igor Sijsling in the first round of the Australian Open in January.

After making the main draw through qualifying, Kokkinakis next meets Turkish ninth seed Marsel Ilhan for a spot in the semi-finals.