(CNN) -- New world No. 1 Serena Williams will take on her older sister and defending champion Venus in the final of the season-ending WTA Championships in Qatar on Sunday as injuries again took their toll on the world's top women tennis players.
Both siblings progressed with semifinal wins on Saturday despite being hampered by physical problems -- Serena again had a thigh heavily strapped, while Venus was hobbling with a dodgy knee.
The effects of a grueling season on the circuit has been exacerbated by the stifling heat and humidity in Doha, with previous No. 1 Dinara Safina's withdrawal early in her opening match due to a back problem setting the tone.
Second seed Serena won the second semifinal by default as Danish teenager Caroline Wozniacki was forced to retire while trailing 6-4 0-1 due to a stomach strain, having been totally sapped by the efforts of her marathon efforts in the round-robin stage.
However, the American has decided to pull out of the U.S. team for next weekend's Fed Cup final against Italy, and will be replaced by Vania King.
Venus showed tremendous resilience to overcome fellow former No. 1 Jelena Jankovic at the last-four stage for the second year in a row, as they went to three sets for the ninth time in 10 career encounters.
The seventh seed triumphed 5-7 6-3 6-4 in two hours and 34 minutes, having to fight back from 2-0 down in the deciding set as both players struggled to hold their own serve.
Venus broke Jankovic for the third time and then managed to hold her service to take a 5-3 lead, and did it again for victory in the 10th game -- which like many in the match went to deuce.
"The crowd really helped me through it," Williams told the WTA Tour's official Web site. "In the first set I couldn't feel the ball, so coming back from not being able to get a ball in to beat such a consistent player like Jelena is great. It's my last tournament of the year so I'm giving 1000 percent."
The 29-year-old has spent longest on court than any other player this week, going to three sets in all four matches for a total of more than 10 hours' playing time.
She made it through to the semis on sets won after winning only one of her three round-robin matches, losing to Serena and Elena Dementieva.
Fourth seed Wozniacki spent six hours and 56 minutes on court in her three opening matches, being crushed 6-2 6-2 by Jankovic on Friday, and had nothing left in the tank for the semifinal.
The 19-year-old U.S. Open runner-up came into the tournament with a thigh problem, then suffered chronic cramp spasms in her epic win over Vera Zvonareva.
She battled gamely before losing the first set on Saturday, then apologized to her opponent and the crowd after conceding the match despite winning the first game of the second set on her serve.
The outcome meant Serena, who has won all four matches this week, will play in the tournament's final for the third time, having won it in 2001.
The Williams sisters teamed up in the doubles after their semifinals, but will not feature in the final after being defeated by Spain's Nuria Llagostera Vives and Maria Jose Martinez Sanchez, who won the tiebreak 10-8 after battling back from losing the first set 2-6 to take the next 6-4.
They will play Zimbabwe's Cara Black and her American partner Liezel Huber who beat the Australian pairing of Samantha Stosur and Rennae Stubbs also 10-8 in a deciding tiebreak.