It was the biggest win of Yuki's career when the third seed formidable Italian, who struggled due to an injured left leg, conceded the match after trailing 1-6 5-5. (Agencies)
Fognini had earlier pulled out of the doubles event for which he was supposed to partner India's veteran star Leander Paes. They duo were seeded second.
Yuki's next opponent will be Canadian world number 32 Vasek Pospisil, who got a walkover from Taipei's Yen-Hsun Lu.
The 21-year-old Indian, ranked 195, is only the third Indian after Somdev Devvarman and Leander Paes to make the singles quarterfinals of this event in Chennai.
Somdev had ended runner-up in the 2009 edition. Paes had reached the semifinal in 1998 when the event was known as Gold Flake Open. Paes had lost to eventual champion Patrick Rafter from Australia.
19-year-old Chennai boy Ramkumar, who came through qualfiers and shocked Somdev in the first round, put up a brave fight against world number 38 Marcel Granollers before losing his second round encounter 2-6 4-6.
To his credit, Ramkukmar continued to play fearless tennis and kept fighting till the last point. He even broke his formidable opponent twice in the second set.
Despite his restricted movements, Fognini was good enough to trouble Yuki with his solid ground strokes, specifically his power-packed forehand.
It was the biggest win of Yuki's career when the third seed formidable Italian, who struggled due to an injured left leg, conceded the match after trailing 1-6 5-5.