The West London R&B star's 'American Boy' - a collaboration with Kanye West - takes the top spot on the strength of download sales only. 'Mercy' slips one place to two.
The week's second highest new entry, Madonna's '4 Minutes', settles for number seven, while Girls Aloud enter the top ten with 'Can't Speak French' at nine.
Elsewhere, there are new entries from Guillemots with 'Get Over It' at 19, Sam Sparro with 'Black and Gold' at 23; and Natasha Bedingfield with 'Love Like This' at 27.
The top ten UK singles in full (click links for reviews):
1. (-) Estelle ft. Kanye West: 'American Boy'
2. (1) Duffy: 'Mercy'
3. (2) Leona Lewis: 'Better In Time' / 'Footprints In The Sand'
4. (4) OneRepublic: 'Stop and Stare'
5. (5) Nickelback: 'Rockstar'
6. (7) Flo Rida ft. T-Pain: 'Low'
7. (-) Madonna ft. Justin Timberlake: '4 Minutes'
8. (9 Utah Saints: 'Something Good 08'
9. (16) Girls Aloud: 'Can't Speak French'
10. (3) H 'two' O ft. Platnum: 'What's It Gonna Be'
> Who's on top of the album chart this week? Click here to find out!