Rafael Nadal took close to two hours to go past Gael Monfils 6-1, 6-7 (5), 6-2 to win in Doha.

Gael Monfils has had knee, back and wrist ailments which not only cost him form and fitness but sapped his morale and motivation.

While Rafael Nadal's rivals have employed Boris Becker, Stefan Edberg, Ivan Lendl, and Michael Chang - Grand Slam winners all - tennis' leading player prefers to work with his uncle Toni.

Murray lost 3-6, 6-4, 6-2 to Florian Mayer, the world number 40 from Germany, who seemed more surprised than anyone at how the contest turned around.

Andy Murray was leading 6-3, 3-0 when the 40th-ranked Mayer started to play more aggressively, going for winners on nearly every shot. Murray's lead evaporated as he only won three of the last 15 games in the one hour, 51 minute match.

Williams collected her 11th title of the year and became the first player to defend the title since Justine Henin in 2007.

Serena Williams, who claims to be exhausted near the end of a long season, reacted with fretful worrying, and sometimes distracting behaviour while struggling to a 6-4, 2-6, 6-4 two-hour win over Jelena Jankovic.

The world No.1 overcame physical difficulties, emotional distress as well as a stubborn opponent in Jelena Jankovic before reaching the final of the WTA Championships with a 6-4, 2-6, 6-4 win.

The fourth-seeded Li Na will play 2011 champion Petra Kvitova, who came from behind to beat Angelique Kerber 6-7 (3), 6-2, 6-3.

Serena Williams was relentlessly focussed against 2011 winner Petra Kvitova, hitting 27 winners and 11 immaculately delivered aces.