Raducanu was already two points away from winning the match at the time. She led 6-2, 5-3, 30-0, having saved three of four break points while converting four of her five opportunities to break Yastremska's serve.
The former US Open champion suffered a huge drop in the world rankings from No.11 to No.83 following her first round exit at Flushing Meadows, so will be looking to rebuild.
Expectations have been high following last year's Grand Slam win, so she now has the opportunity to start fresh.
No. 2 seed Beatriz Haddad Maia of Brazil opened with a 7-6 (3), 6-3 win over Clara Tauson of Denmark. Haddad Maia served up four aces and converted five of eight break points.
No. 5 seed Ekaterina Alexandrova of Russia beat Ukraine's Marta Kostyuk 7-6 (5), 5-7, 7-5, despite committing 14 double faults. And No. 9 seed Anastasia Potapova, also from Russia, needed almost two hours for a straight-sets win over Claire Liu of the U.S., 6-4, 7-6 (5).
Chennai Open
Wild card Karman Thandi rallied to beat No. 8 seed Chloe Paquet of France 4-6, 6-4, 6-3 for an upset victory in her native India.
Thandi, ranked No. 359 in the world, saved five of eight break points. She came back from down 3-2 in the second set, winning four of the last five games in the set to extend the match.
No. 5 seed Rebecca Peterson of Sweden beat Victoria Jimenez Kasintseva of Andorra 6-2, 7-6 (4). Sixth seed Qiang Wang of China outlasted Belgium's Yanina Wickmayer 6-4, 1-6, 6-4, and Canadian seventh seed Rebecca Marino handled Russia's Anna Blinkova 7-5, 6-2.
Other first-round winners included the Czech Republic's Linda Fruhvirtova, Poland's Katarzyna Kawa, Canada's Eugenie Bouchard and Japan's Nao Hibino.
