Serena's mystery illness appears to be over.

Patrick Mouratoglou says Serena Williams is going through a "difficult period."

TennisEarth.com reported that world's No. 1 tennis player Serena Williams is training while on vacation in Croatia with her hitting partner Sasa Bajin. She posted a photo of herself on her Instagram practicing at the Umag Tennis Academy.

She looked fit in the photo as well.

Her message read, "Training on vacy is relaxing. Now time to get back. @pmouratoglou get ready."

Serena Williams' emotional breakdown and subsequent silence is unacceptable

She was referring to her coach and rumored boyfriend or ex-boyfriend Patrick Mouratoglou, who only days ago proclaimed that his student was going through a difficult period.

"Serena is clearly going through a difficult period," Mouratoglou was reported as saying. "But after Wimbledon we had many hours of discussions and we have restarted our work.

"Today, she is in a state of mind where she wants to train hard to come back strong. At no time was there any question of our collaboration ending. ... There were complicated things and it is true that Serena is in a difficult phase, but I'll leave it there."

Serena's health became a topic of discussion following her disturbing behavior in a doubles match at Wimbledon in which she couldn't even bounce a ball or get a serve over the net before retiring. As video of her condition went viral, critics wondered why she even took to the court in the first place and blamed her handlers, including sister Venus, for allowing her to take the court.

The scene came two days after her singles loss to Alize Cornet in the fourth round.

Speculation was rampant that the romantic relationship between Serena and Mouratoglou had ended, contributing to the scene.

Mouratoglou's response to Serena's practice message in Croatia sounded much more chipper.

"@serenawilliams don't tease me! I am already so energized by the new Challenges that we set up as our goals. See you soon, I am ready," TennisEarth.com reported.

Do you think Serena Williams has moved past whatever was bothering her at Wimbledon? Comment below or tell us @SportsWN.