The Londoner enjoyed a bright start as she secured a break point in the first game but she could not convert that and things started to go wrong quickly.
The British number two was swept out of the tournament in just 54 minutes with Kerber winning more than double the amount of points than Keothavong, 55 to 26.
Kerber reached the semi-finals of the US Open last year and also reached the final four at this year's Wimbledon but faces Venus Williams in the second round.
In better news for Britain, Australian-born Johanna Konta, who has represented Britain since being granted citizenship in May, did make the second round after beating Hungary's Timea Babos.
Konta won the match 6-1 7-5 despite being 5-3 down in the second set and will now face Olga Govortsova of Belarus, who downed 29th seed Tamira Paszek 6-3 6-4.
It is Konta's first ever Grand Slam main draw win.
- Johanna Konta