(CNN) -- At times it was almost painful to watch. At one end of the court the world's No. 1 female tennis star playing well within herself; at the other her sister, a long way away from regaining that form and status.
"Venus has had a great week, and honestly, if she hadn't had to play so many matches, it would have been a much tougher match," Serena Williams said after comfortably beating her older sibling on Saturday to reach the final of the Family Circle Cup in Charleston.
The 31-year-old was slightly overstating the rigors of the competition in South Carolina, a tournament that heralded the formation of the women's tour back in 1973 but has this week attracted just two of the world's top-10 players.
Serena is one, and the other -- 10th-ranked Caroline Wozniacki -- crashed out in the quarterfinals on Friday against Swiss No. 63 Stefanie Vogele.
Both Williams sisters won two matches on Friday to set up their first meeting since 2009, but it was defending champion Serena who looked the least affected as she won 6-1 6-2 in just 54 minutes.
"She'll never admit it, but I don't think she was 100%," Serena said of her sister, who was diagnosed with a debilitating autoimmune disease before the 2011 U.S. Open -- a grand slam she has won twice, along with her five Wimbledons.
"But you will never get that out of her. And quite frankly, three matches for her is much tougher than three matches for me. It's definitely not easy -- because I'm struggling, and I can't imagine what she must be feeling."
Venus, seeded fifth this week and ranked 24th, brushed off suggestions that her health issues had hindered her.
"I felt pretty good actually -- it would have been nicer to have more space between the matches, but it did not work out," the 32-year-old American said. "But I played under the same circumstances as everyone else, at least in the top half of the draw.
"I had a chance to win the match. We both had rackets. But I didn't take advantage of my chances. I was controlling the points, but then I would miss.
"I have to improve my game. I have to get in these situations more often. I've been off balance for a long time; I'm trying to regain my balance."
Serena extended her career record over Venus to 14-10, and has now won 70 of her past 74 matches as she seeks to win the tournament -- played on green clay -- for the third time.
"I love playing on this court," said Williams, whose only defeat in her past 22 games on clay came in the first round of last year's French Open. "I love this tournament. I think it's a good clay court.
"Sometimes you play on certain clay courts and they're not good, and they're funny, and you can't really slide, or it's just weird. But this one is a really good one, particularly the last two years."
She will next face former world No. 1 Jelena Jankovic, who ended Vogele's dream run with a hard-fought victory.
The Serbian ninth seed, ranked 18th, triumphed 6-4 6-7 (6-8) 6-2 in two hours eight minutes.
Jankovic also reached the semifinals in Miami last week before losing to Maria Sharapova, who Williams beat in the final for her record sixth title at Key Biscayne.
Meanwhile, in the men's Davis Cup teams competition, the titleholding Czech Republic missed the chance to wrap up a semifinal place after losing in Saturday's doubles against Kazakhstan.
Andrey Golubev and Yuriy Schukin beat Jan Hajek and Radek Stepanek 7-6 (7-2) 6-4 6-3 to reduce the deficit to 2-1 and give the hosts hope ahead of Sunday's reverse singles in Astana.
Argentina took a 2-1 lead at home to France, as David Nalbandian and Horacio Zeballos defeated Julien Benneteau and Michael Llodra 3-6 7-6 (7-3) 7-5 6-3 in Buenos Aires.