LONDON—Kayla Williams probably ruins the curve in her classes at school, too.

The 16-year-old American, who was still competing at the level below elite five months ago, looked like an old pro Saturday in winning the vault title at the world gymnastics championships. Only when she stood atop the podium did she seem overwhelmed, taking several deep breaths as she waited for the gold medal to be put around her neck and fighting back tears.

Just how big a leap was this? Think LeBron James going straight from high school to the NBA — if James had won the NBA title his rookie year.

“This past five months, or however long it’s been, has been the most hectic and crazy,” Williams said, wide-eyed. “But I’ve learned a lot and I finished it off as best I could.”

China, meanwhile, finished things off in its usual fashion, winning three of the five titles Saturday. He Kexin added a world title to her Olympic gold on uneven bars, Zhang Hongtao won pommel horse and Yan Mingyong closed the day with a win on still rings.

Romania’s Marian Dragulescu won his fourth world floor title — not bad for a guy who retired after the Beijing Olympics.

“I was feeling like I can still win medals,” said Dragulescu, who retired because of chronic back and neck pain.

Williams’ rise just doesn’t happen in gymnastics. Newcomers do make a splash at a world championships or Olympics all the time, but odds are they’ve had some international seasoning at the junior level.

Not Williams. She was at the Level 10 championships in May, so far off the radar that her only trip to the Karolyi ranch before the world team was selected was for a developmental camp.

Williams beat the Olympic champion, Hong Un-jong, who fell on both her vaults, as well as Europe’s vault champion, Ariella Kaeslin of Switzerland, who finished second.