By Sudipto Ganguly
MELBOURNE (Reuters) - Not many would have bet on unseeded Americans Danielle Collins and Frances Tiafoe making it to the Australian Open quarter-finals but the duo won their respective fourth-round matches to make it a 'Super Sunday' for their country in Melbourne.
Tiafoe is the sole American left in the men's draw but the women's side could be set for a rich representation with Grand Slam winners Serena Williams and Sloane Stephens and 2017 U.S. Open finalist Madison Keys still to play in the fourth round.
While Serena and Keys play on Monday, 2017 U.S. Open champion Stephens, seeded fifth in Melbourne, will take on Anastasia Pavlyuchenkova in the day's last match.
The 25-year-old Collins had not won a match in her previous five Grand Slam appearances but thrashed second seed Angelique Kerber 6-0 6-2 to reach the last eight in her first main draw showing at Melbourne Park.
Such was her dominance that the fourth-round match lasted only 56 minutes with triple Grand Slam champion Kerber looking completely in awe of her opponent who peppered the Margaret Court Arena with 29 winners.
Collins's coach, Mat Cloer, said his charge was an instinctive player.
"I think sometimes it depends a little bit on the opponent that she's playing but I think she's most comfortable playing offensive tennis," Cloer told reporters.
"Controlling the baseline, really stepping in, taking as much time as she can and she did a great job of that today. Her mentality, just her as a person, she's aggressive.
"So I think you want to let her be herself and let instincts take over and allow her to play as free as possible."
Collins graduated from the University of Virginia in 2016 as the top-ranked college player in the United States. She was ranked 167 in the world in 2017 before climbing to 36 last year.
After losing in the first round at all five Grand Slams previously, she has won four matches at the year's first major.
"I think it's just part of the learning curve to be very honest," Cloer said. "I think it just takes a little bit of time getting familiar with the tour... she didn't know she really belonged there.
"It's just kind of taken a little bit of time and she's put in a lot of work. It's just now the self-belief taking over."
World number 39 Tiafoe celebrated his 21st birthday in style with a 7-5 7-6(6) 6-7(1) 7-5 win over Bulgaria's Grigor Dimitrov, seeded 20th.
Tiafoe was born to immigrant parents from Sierra Leone and his father was a janitor at a tennis centre. Reaching the quarter-finals of a Grand Slam was an emotional moment for the LeBron James fan.
"I mean, yeah, it's crazy, man," Tiafoe said. "Obviously if you guys know anything about me, the story in tennis, I obviously wasn't a normal tennis story.
"The beginning of my career, I was playing for them, trying to do everything for my family. Obviously now I put them in a great place. Now I'm trying to do it for me."
(Reporting by Sudipto Ganguly; editing by Clare Fallon)