"Obikwelu getting out because of a false start this morning made me think too much at the start. So this is why I did not get out of the blocks well," Gay said.
"I know that if I want to beat Powell in the semi-final or in the final, my start will have to be much better. I will have to make some adjustments.
"The speed's there if I just execute my start I will be ready for tomorrow."
Powell described his night's work as 'good'.
"It was as I expected, nothing else," the world record holder said.
"All is going according to my plans."
Both men, who are yet to face each other, are undefeated this year, with Gay the man in form after setting a season-best 9.84 at the US championships in June.
Return to form
Powell has been affected by knee tendonitis and a groin injury but returned to form with a 9.90 last month in Rome, the second best time this year.
Tyson Gay, centre, is aiming for a 100m and
200m double in Osaka [GALLO/GETTY]
The Jamaican burst from the blocks in his evening quarter-final and led all the way before slowing up at the line, with the Bahamas' Derrick Atkins in hot pursuit.
Shortly after in the second heat, Gay took 30 metres to get in front after his slow start but won the race by a distance from Britain's Marlon Devonish (10.13).
Churandy Martina of the Dutch Antilles and Britain's Craig Pickering won heats three and four respectively, with another Britain, Mark Lewis-Francis, and Kim Collins from St Kitts and Nevis among the other qualifiers for Sunday's semi-finals.
Earlier, Gay had raved about the Nagai Stadium surface after coasting through his opening heat in 10.19 seconds.
"It's the best track I've ever been on. I couldn't really run slow, it was kind of hard," he said.
"Words cannot describe the track. I tried to run 10.2 or 10.3 but the track wouldn't allow me to do it."
Gay is aiming to become only the third 100m-200m double world champion after compatriots Maurice Greene in 1999 and Justin Gatlin in 2005.