CHIBA, Japan, Aug 4 (Reuters) - Reigning Olympic champion Helen Maroulis of the United States sounded out a warning to her rivals on Wednesday after scoring convincing victories in her opening bouts of the women"s freestyle lightweight competition to reach the semi-finals. Maroulis, the first female American wrestler to be crowned an Olympic champion when she won gold in Rio, has battled through multiple concussions and post-traumatic stress disorder in recent years before qualifying for the Tokyo Games. The 29-year-old initially struggled to score against Rong Ningning, with the former world champion from China taking a 3-0 lead with a takedown. Maroulis did find an opening for a gut wrench but Rong reversed to make it 4-2 at the break. However, the second period was one-way traffic as the American scored points with a counter tilt, nearly pinning Rong, and two takedowns to win 8-4. Maroulis continued to impress in her quarter-final against Ukraine"s Tetyana Kit, making four takedowns to seal a semi-final berth with an 8-0 victory by technical superiority. The American, who was the bantamweight champion in Rio, will be up against Risako Kawai of Japan, who also won a gold in 2016 but in the heavier middleweight category. In the other semi-final, Bulgarian Evelina Nikolova will face Belarusian Iryna Kurachkina. Nikolova looked to be on the way out after trailing Moldovan Anastasia Nichita 3-0 but put her on her back and made a takedown with just seconds left to win 6-3. Nichita had reached the quarter-finals after a stunning fightback against Nigeria"s Odunayo Adekuoroye. The Moldovan trailed 8-0 but pinned Adekuoroye down and the referee took a close look before awarding her the bout. In the men"s freestyle middleweight category, 2018 world champion David Taylor of the United States beat Belarusian Ali Shabanau 11-0 and followed it up with a commanding 12-2 victory over San Marino"s Myles Amine. Taylor will face India"s Deepak Punia, while the other semi-final will have Iran"s 2016 welterweight gold medallist Hassan Yazdani taking on Russian Artur Naifonov, a three-times European champion.