Serena Williams' Comeback Cut Short Due To Doubles Partner's Injury

Serena Williams' big return to tennis had a lackluster ending ... with the legend being forced to exit the HSBC Championships tournament due to her partner's injury.

44-year-old Williams and Victoria Mboko's journey at the Queen's Club was off to a hot start -- they won in straight sets earlier in the week ... but the 19-year-old suffered a knee ailment in a singles match the next day, which was too much to overcome.

Mboko, 19, had to withdraw from that contest ... and on Thursday morning, the tournament confirmed her doubles match is off, too.

Williams and Mboko were going to face Leylah Fernandez and Laura Siegemund in a quarterfinal match ... so now those two will coast to the next round without breaking a sweat.

This wasn't a one-and-done situation for Williams -- she will also compete next week at the Berlin Open, and admitted she isn't thinking too far into the future ... but doesn't have anything better to do at the moment.