INDIAN WELLS, CA, USA - Unseeded 18-year-old Marketa Vondrousova scored the biggest upset of the BNP Paribas Open so far with a career best win over No.11 seed - a tense, taut 7-6(5), 6-4 affair that was on a knife-edge throughout.

The Czech was swift out of the blocks, leaping out to a 3-0 double break lead and showing off not only her whistle-clean ball-striking but her deft touch, including a confident dropshot-lob combination as early as the second game.

The opening set was not to be decided straightforwardedly, though. Despite holding four points for a 4-0 lead, Vondrousova was pegged back as Konta began to land some scorching forehands. Two double faults - nine would plague the teenager today in total - paved the way to conceding one of her breaks.

Gradually, the Briton began to claw her way back into the set, valiantly retaining her grasp on two deuce service games with excellent serving at crucial times. Her reward came in the eighth game, when Vondrousova - reeling after so many tight tussles - coughed up three easy unforced errors and another double fault to let her second break slip. Rubbing salt into the wound, Konta would capture yet another deuce service game to move ahead for the first time in the next game, saving a break point with an ace.

With the 18-year-old serving to stay in the set, it was time for the momentum to shift again. Continuing to leak mistakes, she fell behind 15-40 - but would save three set points, the second with one of several spectacular down-the-line forehand winners, to haul herself back into contention. A fourth set point came and went for Konta in the eleventh game, fittingly sending the set to a deciding tiebreak.

There, it was the World No.52 who proved more composed. Two double faults and a handful of errant forehands didn't help Konta's cause, but - after 75 minutes in which both player's form had been unpredictable from point to point - it was Vondrousova who seized two of the set's last three points with brilliant forehand winners.

With three long deuce games kicking off the second set, the match remained as closely contested as before. But although Konta would draw first blood, breaking to move up 4-2 when Vondrousova lapsed into error, the Wimbledon semifinalist's 53% first serve percentage was taking away one of the key weapons that has bailed her out when her ground game has gone awry.

Though the former junior No.1 required treatment for a blister in the second set, her strokes were proving more solid - particularly in longer rallies - and she reeled off the next four games to seal the win.

"I would say it's my biggest win," Vondrousova said afterwards of her second career Top 20 victory. "I was trying to fight for every ball, because she was getting better and better, I was trying so hard." Her reward is a place in the third round on her debut in the desert. 

There, Vondrousova, the youngest player in the Top 100, will face the third-youngest player in the Top 100 in 19-year-old Aryna Sabalenka. The Belarusian halted Svetlana Kuznetsova's comeback from wrist surgery at the first hurdle, defeating the three-time finalist 6-4, 6-3 in one hour and 13 minutes.

In another second-round upset, Australian Open semifinalist and No.22 seed Elise Mertens was felled by Wang Qiang 4-6, 6-3, 6-3 - an identical scoreline and result to their only other previous meeting, in the quarterfinals of the ITF $50,000 event in Quanzhou in 2016.

However, No.15 seed Kristina Mladenovic managed to avoid the upset bug. Though the Frenchwoman had never beaten Samantha Stosur in four previous attempts, she was a 7-5, 7-5 victor today in setting up a third-round clash against Wang.