The No.7-seeded Halep was coming off an absolute breakthrough season - she won her first six WTA titles and peaked at No.11 in the world on the year-end rankings - but from the beginning Keys totally outplayed her, pummelling big serves and huge groundstrokes, building a shocking 61 50 lead.
Halep started creeping back into the match, winning four games in a row to close the gap to 61 54 and even holding a point to even the second set at 5-all - but Keys snuck out three points to end it, 61 64.
Keys' win over the World No.11 was the second-biggest win of her career - her best win to date came on the clay courts of Madrid last year, where she upended Li Na, who was ranked No.5 at the time.
Despite the loss, Halep was happy to be back on the court - it was her first match of the new season.
"I'm happy to be back on the court after two months of off-season," the Romanian said. "Last year was a long season for me, so I relaxed a little bit before starting to train again in the off-season, and I practiced two hours a day to get ready for the new year and I'm happy to start the year here."
What are Halep's goals for 2014? "I just want to keep enjoying myself on the court, just like I did last year," she said. "My objective is to stay Top 20 this year and play well at the Grand Slam events."
There were two more first round matches on Day 1, with No.3 seed Sara Errani grinding past good friend and doubles partner Roberta Vinci, 64 62, and Croatian wildcard Ajla Tomljanovic rallying from 3-0 down in the first set and a set point down 5-4 in the second set to beat Daniela Hantuchova, 64 75.
Tomljanovic's win over the No.33-ranked Hantuchova was the second-best win of her career. The best win of her career so far came at Miami last year, where she beat a No.26-ranked Julia Goerges.