On her tour-level clay-court main draw debut in the Spanish capital, the 19-year-old Queenslander outlasted local wildcard Carlota Martinez Cirez 6-2 2-6 6-4 for a third win in three days.
Just five days after her 19th birthday, it meant Joint became the youngest Aussie ever to win a WTA match at 'Masters' level, surpassing Barty who won in the Miami Open in 2017 when she was 20 years and 330 days.
Already having earned notable wins over the experienced duo of Sara Errani and Jil Teichmann in qualifying, the US-born Joint, who's made massive strides over the past two seasons since throwing in her lot with the nation of her Australian dad, came through a roller-coaster first-round affair on Wednesday.
It was the Brisbane-based youngster's first match in a tour-level clay event, but she showed why her record in lesser clay-court tournaments has been so encouraging as she overcame a second-set lapse to dominate the impressive Spanish tour debutant Martinez Cirez.
Brilliant in the opening set, Joint, the second youngest teen in the world's top 100 at No.78, lost her backhand solidity in the second stanza and had to regroup in the decider, eventually cracking Martinez Cirez's serve for the fifth and most decisive time at 4-4.
Joint, who was ranked 269 in the world this time last year, went on to eke out her victory in two hours seven minutes, setting up the chance to make even more waves when she faces American world No.10 Emma Navarro in the second round on Thursday.
Ajla Tomljanovic's hopes of joining Joint in the last-64 ended in a disappointing 6-3 1-6 7-5 defeat to Swiss world No.153 Rebeka Masarova, who's ranked 74 places below her Australian opponent.
With Kim Birrell having been beaten by American Peyton Stearns on Tuesday, it leaves just Joint and Daria Kasatkina, in her second competition as an Australian competitor, flying the flag in the women's draw.
The 14th seed Kasatkina, who had a first-round bye, will open her account against American Alycia Parks, who booked her second-round spot with a 6-3 6-3 win over Andorran Victoria Jimenez Kasintseva.
