As a junior Sugita compiled a win/loss record of 34–19 (and 26–18 in doubles), reaching as high as No. 73 in the combined world rankings in February 2006. He competed in singles and doubles at the Australian Open in both 2005 and 2006, reaching the singles second round of the latter.
In June, Yūichi qualified for Wimbledon by defeating Marco Chiudinelli of Switzerland, 6-4, 6-2, 7-5. It marked the first time he qualified for a Grand Slam after 17 failed qualifying campaigns dating back to 2009. He had previously reached the final round of qualifying at Slams four times, and he dropped decisive sets in three of those matches.