PARIS — Serena Williams was ahead, yes, but hardly at her best, when claps of thunder and a heavy downpour interrupted her third-round French Open match at a critical juncture. So during what turned out to be a delay of more than 2½ hours right before a second-set tiebreaker Saturday against 26th-seeded Kristina Mladenovic of France, Williams met with coach Patrick Mouratoglou. “I spoke 10 minutes, which is far too long.