Stephens, 26, is currently ranked No. 8 in the WTA, reaching the fourth round in this season's Australian Open and the quarterfinals at the Volvo Car Open, where she lost to Madison Keys. She won the 2017 U.S. Open and reached the final at the French Open in 2018.
Altidore, 29, has appeared in five matches (four starts) for Toronto this season, registering five goals and two assists. He also has 110 caps and 41 goals for the U.S. men's national team since first being called up in 2007.
The pair have been dating since 2016.
"She is somebody who gets everything I am going through without having to say anything," Altidore said of the relationship in Oct. 2017, per Martin Rogers of USA Today. "You look at each other and you kind of know right away if one of you is annoyed. She just kind of gets the mood I'm in, as obviously she lives a very similar kind of life.
"In that regard it works really well, there is no stress. She is a successful athlete and I do OK, and we just enjoy our time together when we are together."