Grigor Dimitrov came through a second-round test at the Delray Beach Open as Steve Johnson was eliminated on Wednesday.
Bulgarian fourth seed Dimitrov edged past Damir Dzumhur 4-6 6-4 6-3 at the ATP 250 event played on hard courts in Florida.
Dimitrov twice found himself a break down in the second set and once more in the third before rallying and completing his victory in just under two hours.
The world number 28 won fewer return points than his Bosnian opponent, but booked a quarter-final meeting with Adrian Mannarino.
French eighth seed Mannarino fought past Tatsuma Ito 7-6 (8-6) 6-7 (4-7) 6-3.
The only seed to fall on Wednesday was Johnson, who went down to German Benjamin Becker 5-7 7-6 (7-1) 6-3.
Becker broke just once in the third set on his way to a 4-1 lead before he closed out his victory.
Awaiting Becker is Rajeev Ram after the American overcame Ukraine's Illya Marchenko 6-2 5-7 6-4.