Kaymer reached the pinnacle by reaching the final of the WGC Matchplay tournament at the weekend in Arizona where he lost to England's Luke Donald.
Donald moves up to third, while US Open champion Graeme McDowell.
Tiger Woods, who lost the number one ranking to Westwood on October 31, has slumped to fifth place, his lowest position since before his breakthrough win at the 1997 Masters.
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World top 20:
1. Martin Kaymer (GER) 8.36pts
2. Lee Westwood (ENG) 8.16
3. Luke Donald (ENG) 6.64
4. Graeme McDowell (NIR) 6.44
5. Tiger Woods (USA) 6.32
6. Phil Mickelson (USA) 6.23
7. Paul Casey (ENG) 6.02
8. Rory McIlroy (NIR) 5.65
9. Steve Stricker (USA) 5.49
10. Matt Kuchar (USA) 5.20
11. Jim Furyk (USA) 5.16
12. Ernie Els (RSA) 4.96
13. Ian Poulter (ENG) 4.77
14. Dustin Johnson (USA) 4.47
15. Bubba Watson (USA) 4.37
16. Retief Goosen (RSA) 4.36
17. Robert Karlsson (SWE) 4.34
18. Francesco Molinari (ITA) 4.27
19. Hunter Mahan (USA) 4.23
20. Miguel Angel Jimenez (ESP) 3.86
