Australia's 17th-ranked Nick Kyrgios withdrew from the ATP Washington Open on Wednesday because of a hip injury.
Milos Raonic cranked up the fastest serve of the tournament on Wednesday with a 147mph (236.5km/h) howitzer.
Andy Murray gave an encouraging display in his first competitive match in 11 months after finally recovering from the hip problem that required surgery in January
Roger Federer will play for the 98th title of his career on Sunday against Milos Raonic.
Seven-time major winner Venus Williams suffered back-to-back opening-round exits at the Slams for the first time in her 21-year career.
Dimitrov downed the 17th seeded Kyrgios 7-6 (7/3), 7-6 (7/4), 4-6, 7-6 (7/4) in 3 hour 26 minutes and will face Britain's Kyle Edmund in the quarter-finals.
The 22-year-old was slapped with a USD 3,000 penalty for unsportsmanlike conduct early in his 6-1 6-2 6-4 win over Brazilian Rogerio Dutra Silva in a night match on Hisense Arena on Monday.
The 6-4, 6-2 win was Nick Kyrgios' fourth title on the ATP tour and his first on home soil.
Novak Djokovic, who has been sidelined since July with a right elbow injury, is making his long-awaited return to the game at an exhibition tournament in Abu Dhabi from December 28-30
World number one Rafael Nadal won his sixth title of another remarkable year as he punished a disgruntled Nick Kyrgios at the China Open on Sunday.
World number two Roger Federer handed Team Europe the maiden Laver Cup title after beating Team World's Nick Kyrgios in a super tie-break thriller in Prague.
Maria Sharapova, ranked 146th, played only one US Open tuneup match due to a forearm injury but wore down Simona Halep and Babos to prove she's a threat for a deep run.
Rafael Nadal wore black ribbons during an earlier match in tribute to the victims of twin vehicle ramming attacks in Spain that left 14 dead.
Australia's Nick Kyrgios humbled new world number one Rafael Nadal 6-2, 7-5 to reach Cincinnati Masters semi-finals on Friday.
Gear up for the mouth watering clash as Roger Federer will again battle it out for the summit clash against Rafael Nadal on Sunday.
Should Roger Federer win Friday, there is a chance he will play Sunday's final against long-standing rival Rafael Nadal, who faces Italy's Fabio Fognini in the other semi-final.
Rafael Nadal has never won the Miami Open despite being a four-time finalist and he expects plenty of work to get past Fabio Fognini and make it a fifth appearance.
Federer, who added another chapter to a storied rivalry with Nadal with an epic Australian Open triumph in January, dismantled the Spaniard 6-2, 6-3 to advance.