The Australian Open junior champion a year ago, who entered the season-opening major ranked No. 521, Kostyuk became the youngest player since Martina Hingis in 1996 to win main draw matches at the season-opening major.
World number seven Jelena Ostapenko was knocked out by Estonia's Anett Kontaveit in the third round of the Australian Open here on Friday. "Leonardo is a player with big potential", said Nadal, who won the French and U.S. Opens previous year but had his preparation for Australia delayed because of an injured right knee.
Top seeds continue to progress through the tournament with the likes of second-seeded Caroline Wozniacki, No. 4 Elina Svitolina and No. 7 Jelena Ostapenko leading the way.
In the end, the experienced Frenchman hung on to win the battle 6-3, 3-6, 6-1, 6-7 (4-7), 7-5.
The Denmark star who is in her 12th Australian Open campaign, breathe a sigh of relief when finally getting past the 21-year-old Melbourne Park first-timer.
"She has nothing to lose, so that's why I know she goes just for everything".
World number two Wozniacki produced a great escape against little-known Croat Jana Fett, saving two match points and rallying from 5-1 down in an epic third set to keep her dream of a first Grand Slam title alive. She would then win 8 of the last 12 points to reach the second week at Grand Slam level for the second time after the 2015 Us Open.
Among the seeded men advancing were No. 6 Marin Cilic, who beat Joao Sousa 6-1, 7-5, 6-2, and No. 10 Pablo Carreno Busta, No. 23 Gilles Muller and No. 28 Damir Dzumhur. The 2008 finalist was nearly out of the tournament at 2-5 in the fifth set but claimed five straight games to gain some revenge after Shapovalov ousted him at the US Open. Ivo Karlovic won a marathon in Melbourne for the second straight year.
The top seeds hit 15 winners to set up a second-round clash with Oksana Kalashnikova of Georgia and Varvara Lepchenko of the United States, who defeated Chinese duo Wang Qiang and Wang Yafan 6-4, 6-3. He said, too, that he wouldn't be practicing outdoors Thursday. "That's all. I think it's a health issue, even if I like sometimes (to play in the heat)".
Nadal, a losing finalist past year to Roger Federer, played Leonardo Mayer in the third match on center court.
Keothavong's night got slightly worse when Troicki hit an errant backhand return that slammed directly into the chair umpire's head during the tiebreaker. Nick Kyrgios like Nadal initially failed to serve out the third set against Viktor Troicki on his favored Hisense Arena but wrapped proceedings up in a tiebreak, 7-5 6-4 7-6 (7-2).
A fan had been earlier ejected for yelling out during play, Kyrgios wasn't pleased with excess noise stemming from Keothavong's microphone and at another stage, a helicopter hovered above. Of course it is."The combustible 22-year-old raised expectations of a long stay in Melbourne when he won the title in Brisbane and has appeared mostly composed through two rounds, not surrendering a set".
Alize Cornet beat No. 12-seeded Julia Goerges 6-4, 6-3, knocking the sixth of eight seeds out of that quarter of the draw. Belinda Bencic was unable to follow up her first-round upset of 2017 finalist Venus Williams, Serena's older sister. Kumkhum's best performances have come in Melbourne, having upset twice Wimbledon victor Petra Kvitova in 2014.