Sixteen-year-old Rebeka Masarova beat former world number one Jelena Jankovic on her WTA debut.
Masarova, who lost to Britain's Gabriella Taylor in the quarter-finals of the girl's singles at Wimbledon, beat 31-year-old Jankovic 1-6, 6-4, 6-2 at the Gstaad tournament.
The Swiss is playing as a wildcard in her home country.
The French Open junior champion will face Anett Kontaveit of Estonia in the next round.
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