Serena Williams is excited to face sister Venus in the Lexington Open second round and feels the "tough match" will provide the perfect preparation for the US Open which starts on August 31.

The 23-time grand slam champion made a winning return to the WTA tour on Tuesday but needed three sets to get past Bernarda Pera in Kentucky.

It was the number one seed's first competitive fixture since a Fed Cup qualifier in February and she eventually got close to top gear to win 4-6 6-4 6-1.

A straight sets victory later in the day for Venus Williams over Victoria Azarenka by a 6-3 6-3 score saw her book a last-16 meeting with her younger sister.

Serena Williams told a press conference: "I have a really tough match in the next round, but it is good in general because I haven't played.

"A lot of players have been playing little things and little matches and playing against other players but I've only been training.

"This was really good for me, I was missing shots but I was like 'oh, I should probably hit it that way' so it was trying to get back into the swing of things."

Coco Gauff, who beat Venus Williams at Wimbledon last year, secured her spot in the next round with a 7-5 7-5 success over fellow American Caroline Dolehide.

Fifth seed Yulia Putintseva also progressed into the second round with ease after a 6-0 6-4 win over Aila Tomlianovic.

But former US Open champion Sloane Stephens suffered an upset at the hands of Canadian qualifier Leylah Annie Fernandez, with the teenager beating the American 6-3 6-3.

At the Prague Open, Simona Halep needed seven match points to finally beat Polona Hercog in a hard-fought three-setter.

The Romanian, who is the top seed at the event, progressed 6-1 1-6 7-6 (3) after two hours and 31 minutes.

Halep told wta.com: "I knew it was going to be difficult. All our matches were very tough and very long.

"She knows me pretty well, we played in juniors. She changes the rhythm, she plays drop shots, a little bit softer then harder, so it was tougher for me to be in the point."

Second seed Petra Martic joined Halep in the second round after a 7-6 (2) 6-3 triumph against Vervara Gracheva.

Yet Anastasia Pavlyuchenkova and Anastasija Sevastova were two seeds to exit after defeats to Arantxa Rus and Irina Begu respectively.