Japanese qualifier and Belgian wildcard David Goffin upset seeded players to move into the second round of the ATP Chennai Open on Tuesday.
Sugita beat eighth-seeded Olivier Rochus of Belgium 6-3, 6-4 and Goffin stunned last year's finalist and seventh-seeded compatriot Xavier Malisse 4-6, 6-2, 6-3 in the $450,000 tournament.
"This is my best win ever," the 23-year-old Sugita told AFP. "We Japanese love a good fight in sport and this win has given me a lot of confidence.
"We are both baseline players, so there were a lot of rallies. To win in straight sets is a good start to the year."
Sugita next plays Yen-Hsun Lu of Taiwan, a 6-3, 6-3 winner over Dutch qualifier Thiemo de Bakker.
The 49th-ranked Malisse, who breezed through the first set, complained of illness after Goffin ran up a 5-2 lead in the second set and did not recover after that.
"I think he ate something bad and had a stomach problem," Goffin said of Malisse, who returned to the hotel as soon as the two-hour match ended.
"I was only concentrating on my game and am very happy to beat a player of his stature. I had never played him before, only watched him on television."
Goffin will now take on Andreas Beck of Germany for a place in the quarter-finals. Beck overcame Canadian Vasek Pospisil 6-3, 7-6 (10/8) in the first round.
With sixth seed Fabio Fognini having crashed out on Monday, the tournament has lost three seeded players in the first two days of action at the Nungambakkam tennis stadium.
The top contenders, including number one seed Janko Tipsarevic of Serbia, will feature on Wednesday after being handed first-round byes.
Tipsarevic, who broke into the top-10 for the first time last year and finished the season ranked ninth, will take on Indian wildcard Yuki Bhambri in the second round.
The Serb's main challengers for the title are world number 10 Nicolas Almagro of Spain, defending champion Stanislas Wawrinka of Switzerland and the big-serving Milos Raonic of Canada.
Frenchman Edouard Roger-Vasselin, who was drafted into the main draw as the lucky loser after India's Somdev Devvarman pulled out with a shoulder injury, moved into the second round.
Roger-Vasselin defeated compatriot Eric Prodon 6-4, 6-3 to set up a meeting with Wawrinka.
Victor Hanescu of Romania ousted American Sam Querrey 6-3, 6-4 and will take on Raonic for a quarter-final berth.