Venus Williams came into the tournament on a poor run of form
Elena Dementieva battled back to beat defending champion Venus Williams on day one of the season-ending Sony Ericsson Championships in Doha.
The Russian recovered from a set and a break down to win 3-6 7-6 (8-6) 6-2 in the first match of the White Group.
In the same group, Serena Williams beat Svetlana Kuznetsova 7-6 (8-6) 7-5 in the final match of the day.
Belarussian Victoria Azarenka earlier enjoyed a superb debut, beating Jelena Jankovic 6-2 6-3 in the Maroon Group.
Caroline Wozniacki and Dinara Safina complete the eight-player field and they meet on Wednesday.
The season finale is split into two groups of four, with the top two in each group making the semi-finals and the eventual champion taking home US$1.55m.
Serena Williams hopes to regain the number one spot from Safina - the American must win one more match than the Russian to do so.
Twelve months ago Jankovic came into the tournament as the world number one, but a mark of her slump in form is that this time around the Serbian is the lowest ranked player of the eight on show.
Sixth seed Azaranka, in contrast, has made the breakthrough into the top 10 in 2009 and was much the steadier in the opening match.
Jankovic, 24, was in desperate form, dropping serve twice as the first set slipped away and, after recovering an early break in the second, handing it straight back with a succession of errors.
Azarenka came through a lengthy game eight on her way to sealing victory in one hour and 22 minutes and taking charge of the Maroon Group.
"I basically gave her everything," said Jankovic. "I beat myself. I was just a shadow of myself."
Azarenka took control of her debut match in the season-ending event
Azarenka, 20, responded: "I'm glad she gave me the match and that she wasn't in the mood.
"It worked out pretty well for me. I'm just happy with the way I started. I haven't played in a tournament for a while. I felt pretty good out there."
Venus Williams looked in complete control when she led the second match of the day by a set and a break, with Dementieva throwing in 10 double faults during the second set.
Fortunately for the Russian, Williams also lost her way and made 27 unforced errors to allow Dementieva into a tie-break which she narrowly edged.
And the Olympic champion dominated the decider as an increasingly frustrated Williams could do nothing to prevent an opening defeat.
The American later refused to blame the humid conditions, saying: "I'm a Florida girl. It gets hot there too. I had plenty of opportunities to open a big lead, but I just didn't get enough balls in the court."
Dementieva said: "In the end I was very emotional, very positive. When you play against a player like Venus you can't put pressure on yourself but you have to fight or every point."
Serena Williams avoided the same fate as her sister as she came through a scrappy encounter with Kuznetsova, but she had to save two set points in the first set tie-break and required heavy strapping on her ankle before the second.
The Australian Open and Wimbledon champion was still strong enough to edge past the French Open winner for a straight-sets win.
And of her ankle, Williams said: "I rolled it in the tie-break but it came out well and I just had to get it re-taped."
Wednesday order of play (start 1400 GMT):
Victoria Azarenka (BLR x6) v Caroline Wozniacki (DEN x4)
Dinara Safina (RUS x1) v Jelena Jankovic (SRB x8)
Serena Williams (USA x2) v Venus Williams (USA x7)