A payments platform created by India’s largest retail banks surpassed a billion transactions in October, a milestone that affirms the tremendous growth of services offered by U.S. giants from Walmart Inc. to Amazon.com Inc. and Google.

Indian digital payments took off when the government pushed demonetization in 2016, invalidating most of the country’s high-value currency notes in a move to curb corruption and push Indians away from cash. The Unified Payments Interface or UPI has now surpassed a 100 million users three-and-a-half years after its launch, thanks to booming smartphone use and wireless data rates among the lowest in the world.

Amazon and Google now vie with local startup Paytm, Walmart-PhonePe and a host of other players in a digital payments market forecast to quintuple to $1 trillion by 2023.

“UPI has had the fastest acceptance rate not just among payment platforms but digital platforms of any kind,” said Dilip Asbe, chief executive officer of the National Payments Corporation of India, which groups the lenders that developed the system. “We aim to expand the UPI base to 500 million users in the next three years.”

UPI sports an open architecture allowing digital wallet apps, payment banks and startups to link freely to its platform. That’s a contrast to the closed systems of China’s dominant services, WeChat and Alipay. It simplifies transactions between apps and banks linked through a biometric ID system called Aadhaar. Regulated by the central bank, it allows the instant transfer of funds between bank accounts through a mobile device using a simple virtual handle, without sharing bank or personal details. UPI’s creators now want to take the platform beyond the country’s borders and allow payments in places like Singapore or the Middle East with a dense population of Indian expatriates.

Global tech companies are racking up enormous losses as they battle for a slice of a market forecast to grow to $1 trillion by 2023 from about $200 billion early this year, according to Credit Suisse. Between SoftBank-backed Paytm, Walmart’s PhonePe and Amazon Pay alone, collective losses totaled over $1 billion in the financial year ending March, a 167% spike from the previous year, according to the Economic Times. Losses are mounting as they drive further into India’s smaller cities and towns, luring merchants and first-time phone users with cashbacks and financial sweeteners.

All those players aim to get into services like lending as their next move, so they can start making money, said Arnav Gupta, a New Delhi-based digital strategy analyst at Forrester Research. “The market potential in a country of nearly 1.4 billion Indians is massive.”

But competition is set to worsen. Facebook Inc.’s WhatsApp is seeking final approval for the full-fledged countrywide launch of its payments service, which has been in a long-drawn pilot phase with just 1 million users. Experts predict that the entry of WhatsApp into the arena will mark another inflection point because of an estimated user base of more than 400 million.

WhatsApp Payments will be launched “soon” in India, Facebook founder Mark Zuckerberg told investors in a post-earnings conference call this week. WhatsApp is said to be seeking final approval after storing user data on local servers, as mandated by the central bank.