27... Bg3? Based on a tactical oversight. After a move like 27... a5 Black doesn't seem to have many problems, since the line 28. Rxd5 exd5 29. Nf5 fails now to 29... Bd8! 28. Rxd5! exd5 29. Nf5 Threatening mate and taking the bishop. 29... Ke8 30. Nxg3 Kd7 31. Bd4 Rbb8 32. Ra1 1-0