Miami, United States: Top-seeded Juan Martin del Potro reached the quarter-finals of the ATP Delray Beach Open Thursday with a 6-4, 6-4 victory over American Reilly Opelka.
Del Potro, playing his first tournament since breaking his right kneecap at the Shanghai Masters in October, withstood 18 aces from the 21-year-old American, who was coming off his first ATP Tour title at the New York Open on Sunday.
Del Potro, the 2011 Delray Beach champion, converted the only two break points of the match and held serve at love in the final game to finish off Opelka in 1 hour and 13 minutes.
The world number four next faces American Mackenzie McDonald, who defeated Spain's Guillermo Garcia-Lopez 7-5, 6-4.
Fourth-seeded American Steve Johnson advanced with a 7-5, 7-5 victory over Italian Paolo Lorenzi.
He'll take on Moldova's Radu Albot, who survived a mid-match surge from Nick Kyrgios to advance 6-2, 3-6, 6-3 in a match marred by the Australian's outbursts at a fan and linesman.
In Friday's other quarter-finals, second-seeded John Isner will take on France's eighth-seeded Adrian Mannarino and British qualifier Daniel Evans faces sixth-seeded Italian Andreas Seppi.