Melbourne - Petra Martic celebrated her 27th birthday on Friday by holding off a gritty challenge from Thai qualifier Luksika Kumkhum to reach the last 16 of the Australian Open for the first time.
Martic battled fierce heat and a succession of powerful winners from her opponent to prevail 6-3, 3-6, 7-5 after two hours and 10 minutes on Rod Laver Arena.
World number 81 Martic has reached three other Grand Slam fourth rounds but was made to battle all the way to get to her first in Australia.
The 124th-ranked Kumkhum went for broke throughout a ding-dong battle, powering a remarkable 44 winners past the Croatian, who had just 18 of her own.
"She raised her game and whenever you give her pace on the ball she hits winners all over the place," said Martic.
"Two days ago I was already struggling so it was really tough today," added Martic who was clearly suffering after a succession of long rallies towards the end of the third set as the mercury rose towards a forecast 42 degrees Celsius.
She will face Belgium's Elise Mertens or Alize Cornet of France for a place in the quarter-finals.
Results from the Australian Open on Friday:
Kyle Edmund (GBR) bt Nikoloz Basilashvili (GEO) 7-6 (7/0), 3-6, 4-6, 6-0, 7-5
Petra Martic (CRO) bt Luksika Kumkhum (THA) 6-3, 3-6, 7-5
Denisa Allertova (CZE) bt Magda Linette (POL) 6-1, 6-4
Elise Mertens (BEL) bt Alize Cornet (FRA) 7-5, 6-4