The eighth seed was on song was the start with her movement, anticipation and clean hitting.
Sun competed well but struggled to make a dent on the Gracheva serve and was often under pressure on her racket.
While Sun has around only 10 matches at this level, Gracheva is a seasoned pro who has been established inside the top 100 for the last five years. She has reached a WTA final and made the third round of a grand slam on three separate occasions.
That experience showed. She only dropped two points on serve throughout the first set, as Sun couldn’t make a dent, while struggling from the service line herself.
The second set was tighter, though Gracheva fended off the two break opportunities she gave up.
Sun continued to have issues on serve – landing only 48 per cent of first serves across the match – which exacerbated the pressure. Gracheva grabbed the crucial break at 4-4, with a sizzling passing shot down the line, before yelling with joy.
The French player blew one match point but converted the second, with a forced error from Sun.
In the first match of the day seventh seed Petra Martic had a routine 6-2 6-2 win in 74 minutes over China’s Yue Yuan.
The Croatian world No. 41 was completely dominant – off the back of her strong serve – while Yuan (77) couldn’t get any flow.
Martic faces American fourth seed Emma Navarro in Friday’s quarter-finals, while Gracheva has to take on top seed Coco Gauff.
Michael Burgess has been a sports journalist since 2005, winning several national awards and covering Olympics, Fifa World Cups and America’s Cup campaigns.