Sergent claimed her first world title at the 1987 IAAF World Cross Country Championships, beating Liz Lynch by two seconds. She also led the French women to second in the team rankings behind the United States (led by Lynn Jennings). Her debut at the 1987 World Championships in Athletics followed and she narrowly missed the final, finishing eighth in her heat. She began the 1988 season with wins at the Cinque Mulini cross country meeting and the French Championship race. At the World Cross Championships in Auckland she failed to retain her title but won a medal for the third year running, taking individual and team bronze medals in the long race. On the track she set national records in the 3000 m and the 10,000 metres. She was a finalist in both events at the 1988 Seoul Olympics, finishing 12th and 19th respectively. That year she ranked second over 5000 m at the IAAF Grand Prix Final, finishing as runner-up to Britain's Liz McColgan.