Melbourne, Feb 16 : Japanese world number three Naomi Osaka eased into the semi-final of the 2021 Australian Open with a straight sets win over South Korea's Hsieh Su-Wei on Tuesday at the Rod Laver Arena. The three-time Grand Slam champion, seeded third for the tournament, beat Hsieh 6-2, 6-2. Hsieh, 35, is the oldest woman in the Open to have debuted in the quarter-final of a Grand Slam. Osaka was thrown off initially by the Korean's unorthodox style of play but eventually overcame her struggles to dominate the match. Osaka now faces either American 23-time Grand Slam winner Serena Williams or Romanian second seed Simona Halep who play their quarter-final match later on Tuesday. This is the first time since she won the tournament in 2019 that Osaka has reached the semi-finals at the Australian Open. She has gone on to win a Grand Slam every time she reached the semi-finals. /IANS