Linz - American teenager Coco Gauff advanced to the first WTA semi-final of her career on Friday with a 7-6 (7/1), 6-4 defeat of top seed Kiki Bertens at Linz.

The 15-year-old became the youngest woman to reach a semi-final at this level since Nicole Vaidisova won the Tashkent title in 2004. Gauff plays Andrea Petkovic on Saturday for a place in the final.

The 75th-ranked Petkovic booked her first semi in a year, defeating Viktoria Kuzmova of Slovakia 6-4, 6-1.

Gauff earned her place in the field as a lucky loser from qualifying as Greek Maria Sakkari withdrew injured to start the week.

The Floridian, still under WTA age restrictions, is playing her fifth Tour-level main draw of 2019 after reaching the Wimbledon fourth round and making the third round at the US Open.

The Linz week has been a breakthrough for the world No 110 who will crack the top 100 on Monday, when she travels to Luxembourg for the last event of her season.

She is already assured of a spot in the Australian Open field in January with her rise in the rankings.

Gauff maintained control throughout over the eighth-ranked Bertens to earn her first victory over a top 10 player.

"There have been a lot of firsts this week," Gauff said. "I'm really happy and proud of how I'm playing.

"Kiki was playing well and there was just a difference of one or two points.

"I'm glad I was able to come out on top. I've really gotten a lot of support from the crowd," she told a sellout audience in this upper Austrian regional capital.

Gauff was never threatened by her more experienced opponent, who owns titles this season at St Petersburg and Madrid and reached the Beijing semi-finals last week before returning to Europe.

Gauff stepped up to win the opener in a tie-break and broke for 2-1 in the second.

The match ended on Gauff's second match point as Bertens sent a return wide.