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.
Li Na is already the first Chinese singles player to have won a Grand Slam title and to have reached the world's top five.
The world number two from Belarussia admits to having become over-tired near the end of the season, and once the intelligent and strongly-focussed Jankovic had tenaciously clung to her break to take the first set she began to make steady progress.
The world number one from the United States took little more than an hour and allowed only four games to Angelique Kerber, who beat her in Cincinnati last year but who was now outplayed almost from the moment she dropped serve in her opening service game.
The 26-year-old Russian -- presently number three in the world -- had had to withdraw from the US Open on the eve of the tournament because of the injury and owing to a previous hip problem has played just one match since a second round exit at Wimbledon in June.
Djokovic put away Lukas Rosol of the Czech Republic 6-0, 6-3, Nadal claimed victory over Santiago Giraldo and Serena lost her serve twice to Francesca Schiavone before winning the battle.
The world number five booked her place in next month's WTA Championships in Istanbul featuring the top eight women's players.
The unseeded Italian, 33, seeking her second title of the year, overcame France's Virginie Razzano in three sets 7-6 (7/5), 3-6, 6-3 to reach the last eight for the first time since June.