Tennis: Coco Gauff overcomes tough test from Marta Kostyuk to book Australian Open semi-final

US Open champion Coco Gauff has come through a huge test at the hands of Ukrainian world No.37 Marta Kostyuk 7-6(6) 6-7(3) 6-2 to reach the semi-finals of the Australian Open for the first time.

The fourth-seeded American will rarely play as badly and still progress, but prevailed after more than three hours on a blistering hot Rod Laver Arena to fulfill coach Brad Gilbert's famous maxim by "winning ugly".

Playing her final Grand Slam as a teenager, Gauff had swept into the last eight on a nine-match winning streak, as she looked to back up her first major success at New York last year.

Kostyuk got right in the ASB Classic champion's face from the off and stormed to a 5-1 lead, serving twice for the opening set, as well holding a setpoint on Gauff's serve.

Marta Kostyuk.
Marta Kostyuk. Photo credit: Getty Images

The American knew she was up against it and, problem-solving on her feet, battled her way back into the contest by rattling off five successive games to serve for the set herself.

Kostyuk had looked increasingly frequently at her coach, as her confidence waned, but she earned three breakpoints off Gauff's forehand and forced the tiebreak, when the American double-faulted.

The Ukrainian was revived by treatment on blisters on her feet before the tiebreak, but blew her second setpoint, allowing Gauff, 19, to come racing into the net to go a set up.

The players traded breaks throughout a second set featuring some lengthy rallies, but Gauff was able to edge ahead and serve for the match at 5-3.

Again, a combination of Gauff's frail second serve and Kostyuk's ability to conjure up winners - she fired 39 across the contest - allowed the Ukrainian to get back on serve and then level up the contest at one set all.

Gauff found the fix, as Kostyuk tired in the third set, by ramping up the pace of her first serve and backhand, but was broken when serving for the match for the second time.

She finally got over the line at the third time of asking to move into a semi-final against either Aryna Sabalenka - a rematch of last year's Flushing Meadows final - or Czech ninth seed Barbora Krejcikova.