(CNN) -- Venus Williams will meet younger sister Serena in the semifinals of the Sony Ericsson Open in Miami after a comfortable 6-1 6-4 win over Iveta Benesova on Wednesday.
Venus Williams will go into her semifinal clash with sister Serena in Miami in better form.
Venus is gunning for her first title at Key Biscayne since 2001 and will go into the 20th All-Williams meeting full of confidence after casting aside her 26th-seeded Czech opponent in quick time.
By contrast, top seed Serena was far less convincing as she recovered from a set down, before also enduring a second-set tie-break, to eventually overcome the plucky challenge of Li Na 4-6 7-6 6-2.
The Chinese right-hander quickly established herself in the match by racing to a 5-0 lead against a lethargic looking Serena.
The world number one eventually snapped out of her early malaise to grab back two breaks of serve, but she had given her opponent too great an advantage and Li finally served out at the third time of asking.
Li continued to keep pace with Serena in the second set, and could even have taken a potentially decisive break at 4-4 only to fail to take her chance.
Off-form Serena allowed her 27-year-old opponent another break opportunity at 5-5, which she also failed to take, before the set went to the tie-break.
Finally the world number one stamped her authority as she dropped just one point in the tie-break. From there Li's hopes were undermined by an increasing error rate that allowed Serena off the hook as she claimed the decider 6-2.
Afterwards Serena, who is chasing a record sixth title in Miami, told reporters: "I definitely wasn't moving my feet at all in the first set.
"I just wasn't doing anything that made a lot of sense at all. I'm a little disappointed. I don't think I played great, but I'm glad to have got through. I definitely played better in the third set, which was good, because I needed to play pretty well to win the match."
Serena will now play her sister in the last four knowing she will need to improve. She also must overcome a losing 10-9 career record against Venus and has lost three of their past four meetings, including last year's Wimbledon final.
In the men's event, third seed Novak Djokovic battled a stomach complaint late on to beat Jo-Wilfried Tsonga 6-3 6-4 to book a semifinal showdown with Roger Federer.
The Serb looked on course to end a four-match losing run against Tsonga, when he raced through the opening set before breaking in the fifth game of the second to go 3-2 ahead.
But two games later the world number three's problems became evident as he called a medical time out to help deal with stomach cramps.
After the break in play Djokovic was clearly still hampered as he missed 14 first serves and while it gave Tsonga a sniff of victory, the Serb gritted his teeth to claim the match with a forehand winner down the line.
Meanwhile, second seed Federer secured his place in the last four after a battling victory over home favorite Andy Roddick.
The Swiss overcame the in-form American, as well as some partisan home supporters, to go through 6-3 4-6 6-4.