His game deteriorated after losing a second-set tiebreaker and despite medical treatment, he had trouble getting around the court as Dimitrov charged to victory.
Dimitrov lost to Rafael Nadal in the quarter-finals in Melbourne three years ago and last year he fell to Federer in the third round.
The tennis star began dating rapper Common in 2008, a relationship which lasted for two years and was later rumoured to have continued years later.
Williams was back in Boston on Wednesday night for Fed Cup matches against Belarus.
Serena Williams has many times been ranked number one in women's singles tennis, but that doesn't mean that she, herself, has to be single!
Facebook App: Open links in External Browser There is a specific issue with the Facebook in-app browser intermittently making requests to websites without cookies that had previously been set.
This appears to be a defect in the browser which should be addressed soon.
The first high profile romance rumours she was involved in was back in 2002 with American pro football player, Keyshawn Johnson, who was almost 10 years her senior.