It happened again on Thursday at the Dubai Championship, where Serena took out Jankovic 6-2, 6-2 to advance to the semifinals but not before a rather testy final game that saw both players yell across the net at each other.
Jankovic was visibly upset by the time Serena was taking between points, trying to serve points faster and faster with Williams holding her up a few times.
You'll see in the video below that Williams holds up her racquet as a sign to wait so she could cough, and that was the final straw for Jankovic, who walked away from the service line and apparently yelled "you can take more time, it's yours" to Serena sarcastically.
Williams fired back, "Do you want to do this again, Jelena" to the delight of the crowd, drawing back memories to similar situations at the 2013 Family Circus Cup and the 2010 Italian Open, when time was another issue between players.
After Williams broke Jankovic the two met at net, and Williams looked to try to clear things up with Jankovic, saying afterwards that she just wanted to make sure everything was okay between the two after the match was completed.
''I told her at the end, 'Look, I didn't mean anything. I'm sorry if I played slow,''' Williams said. ''She was like, 'No, it was more like they say (I) play too fast and then too slow.' She said she couldn't get it right.
''I was like, 'Look, are we cool because I'm cool with you?'''
Williams now moves to 9-4 in her career against Jankovic, so maybe she wasn't as annoyed at Serena as she is at her inability to take her down, losing her last five matches to the No. 1 player in the world.
Serena moves on to play Alize Cornet in the semifinals on Friday.