By Syndicated Content Feb 17, 2021 5:11 AM MELBOURNE (Reuters) - Novak Djokovic is one of the best-known sportsmen in the world but his semi-final on Thursday will be against a player he confessed he was unaware of until this year's Australian Open. The top-ranked men's player would not be the only one in the tennis circuit who would not have previously heard of the 114th-ranked Russian Aslan Karatsev, 27, before 2021. But in the past two weeks on Melbourne Park's blue hardcourts, the qualifier has ensured he has the firm attention of the tennis world by becoming the first man in the open era to reach the last four in his maiden Grand Slam.