MELBOURNE, Jan 23 (Reuters) - World number one Ash Barty will have to overcome the formidable Amanda Anisimova to keep alive her hopes of becoming the first Australian since 1978 to win their home Grand Slam when the two meet in the last-16 at Melbourne Park on Sunday. Barty and defending champion Naomi Osaka had been poised to meet in a heavyweight clash in the fourth round, but Anisimova played spoiler, edging out the Japanese four-times major winner to extend her winning streak to eight matches this year. The unseeded American could cause problems for Barty, who won their only previous meeting -- at the 2019 French Open -- but will be wary of Anisimova"s power from the baseline, which was evident against Osaka as she fired 46 winners. On the men"s side, Rafa Nadal will continue his pursuit of a record 21st major against fellow left-hander Adrian Mannarino, while German world number three Alexander Zverev faces a tricky test in the form of Canadian 14th seed Denis Shapovalov. Nadal, who dropped a set against Karen Khachanov but looked in good rhythm, will start as the overwhelming favourite against Mannarino, but the Frenchman will also fancy his chances following upset wins over Aslan Karatsev and Hubert Hurkacz. Fourth-seeded French Open champion Barbora Krejcikova will look to halt the charge of Belarusian Victoria Azarenka, who is seeking a third crown in Melbourne, while Greek fifth seed Maria Sakkari plays American Jessica Pegula for a quarter-final spot.