Williams withdrew shortly before her highly-anticipated fourth-round clash with Maria Sharapova at Roland Garros on Monday, citing a right pectoral injury that prevented her from serving. The American is expected to undergo an MRI scan in Paris on Tuesday but had seen one doctor earlier in the day.

"So today has been interesting already," Serena said via a story posted on her Instagram channel. "I just finished one exam with one doctor, and the news is actually super exciting, so I'm really happy about that.

"I'm having an MRI soon, but I wanted to keep you guys totally updated. Thanks for the support and love and understanding."

Williams, a 23-time Grand Slam champion, was competing at a major tournament for the first time in 16 months -- and for the first time since giving birth to her daughter, Olympia, in September.

"It's been incredibly frustrating, but also I'm excited because I feel like I've been playing really, really well and I can only go up," Williams said on Instagram. "And I'm looking so forward to going up. And hopefully you guys will be there to continue to cheer me on. I love you guys."