Williams defeated Jankovic 3-6, 6-0, 6-2 Sunday for her second consecutive Family Circle Cup title. Momentum swung for good at the start of the second set, when Williams said Jankovic was serving too quickly, before she was ready.
Jankovic disagreed, but the bickering disrupted her concentration and her play. Williams won six straight games and 12 of the final 14.
"I don't know what the turning point was," Williams said. "I mean I think after that I just got really relaxed and I was like, 'Honestly, Serena, you've got to kind of chill out and not get crazy.'"
Instead, the world's No. 1 player displayed a brand of unstoppable tennis she's shown often, and especially at this event. She became the first woman to capture three titles since the Family Circle moved from Hilton Head Island to Charleston in 2001.
"Definitely a really cool accomplishment, really cool, especially at this particular tournament that has been around for so long," she said.
It looked for a while that Jankovic was on her way to accomplishing what no one had done in a complete match since Justine Henin in the 2003 finals -- Williams withdrew twice because of injury in that span -- and defeat Williams on the Family Circle's green clay.
Jankovic broke Williams twice on the way to winning the first set, the only one Williams had lost this week.
"The key was I served very well and made a lot of first serves in, so she couldn't attack it," Jankovic said.
Jankovic, a former No. 1 player, had two chances to take a 1-0 lead in the second set when things unraveled. She served while Williams held her racket in front to signal she wasn't ready. During the next changeover, Jankovic asked chair umpire Kader Nouni how long she needed to wait before serving.
"Until I'm ready," Williams shot back.
Williams won the next eight straight points to take a 2-0 lead.
Jankovic acknowledged she should have shaken off the exchange and continued playing as she had. Instead, Williams surged back.
"I managed to lose them," Jankovic said. "So, of course, she's going to go up and feel much better and she is again in control. So that was my mistake, and it was unfortunate for my side."
Jankovic, though, felt she did nothing wrong.
"She should follow the return and not the opposite, like she said, that I've got to wait for her to be ready," Jankovic said. "That's not true."
It was the second straight tournament in which Williams dropped the opening set in the final before digging in for victory. A week ago, she lost 6-4 to Maria Sharapova and then won 12 of the next 15 games to win the Sony Open.
Jankovic tried to bounce back in the decisive set and was ahead 1-0. But Williams took the next three games to regain control. Williams pounded a 110 mph serve that drove Jankovic wide and then put away the ball near the net to take the match.
Any hard feelings didn't seem to last.
Jankovic met Williams with a big smile at the net as the two traded good wishes. When the stadium announcer asked the crowd to acknowledge Jankovic's strong play as she left, Williams joined fans in applauding.
For Williams, it was her 49th career singles title, moving within four of Monica Seles for ninth place on the WTA's list. It wasn't the easiest run to the top, though. Midweek rain wiped out much of Thursday's schedule, meaning Williams had to win twice on Friday to make to the semifinals.
Once there, Williams had to face big sister Venus and responded with a 6-1, 6-2 victory, the most one-sided match in the siblings' long rivalry.
Williams earned $125,000 for her third Family Circle Cup crown, also winning in 2008 and '12.
She had hoped last year's clay-court victory on Billie Jean King Court would spur a big run to a second French Open crown. Instead, Williams was knocked out at Roland Garros in the opening round by then 111th-ranked Virginie Razzano.
"This year," she said jokingly after the match, "my goal is to win a match at the French Open."