PARIS – Serena Williams went from a 72-minute struggle of a first set that required a tiebreaker to resolve in her first-round match at the French Open to a lopsided second set in which she did not drop a game.“I just need to play with more confidence,” Williams said, by way of explanation for how her 7-6 (2), 6-0 victory over 102nd-ranked Kristie Ahn unfolded Monday, “like I’m Serena.”Well, yes, she was more herself for the latter half of the contest, which by the end really wasn’t much of a contest.And this is how Ahn described the sensation of being across the net from that version of Williams, the one where she is at her very best. “It feels like you’re trying to push a runaway train in the opposite direction,” Ahn said.