ROME (AP) Singles was the easy part for Venus Williams on Monday. The difficult part came with younger sister Serena in doubles.

After Venus comfortably beat CoCo Vandeweghe 6-4, 6-3 in the first round of the Italian Open, the Williams sisters lost to Andreja Klepac and Katarina Srebotnik of Slovenia 6-1, 7-5.

It was the first time the sisters played doubles together since the 2014 U.S. Open. They are preparing for an attempt at a fourth Olympic gold medal in doubles in Rio de Janeiro.

''We didn't play our best at the same time,'' Venus said. ''That's not typical. Usually one of us is playing well. That helps the team a lot. Two of us are playing well, it usually goes our way.''

In other first-round matches, Sara Errani was slowed by physical problems in losing to Heather Watson of Britain 6-4, 3-6, 6-0, and Lucia Safarova of the Czech Republic defeated former French Open champion Francesca Schiavone 6-3, 6-2.

On a rough day for the home players even in the men's tournament, Fabio Fognini of Italy was beaten by Guillermo Garcia-Lopez of Spain 6-1, 7-6 (2).

Also, Milos Raonic overcame two breaks of his serve to get past Italian wild card Marco Cecchinato 6-4, 4-6, 6-4.

Raonic will next face Nick Kyrgios, who eliminated Salvatore Caruso, another Italian wild card, 6-1, 6-2.

Seventh-seeded Jo-Wilfried Tsonga withdrew due to a muscle problem and was replaced in the draw by fellow Frenchman Lucas Pouille. He doesn't have much time to recover before the French Open, which begins in two weeks.

''It's not something good to be a little bit injured before this event,'' Tsonga said. ''I hope I will have the chance to recover.''

The big four - Novak Djokovic, Roger Federer, Andy Murray, and Rafael Nadal - are all entered.

Playing two matches in one day could have been particularly taxing for Venus, who has Sjogren's syndrome, a condition that can cause joint pain and sap energy.

''I haven't done that in a long time. But I knew what was going to happen going into it. Even if you get tired, you have to try to ignore it. I felt OK,'' she said. ''Hopefully I'll have that problem for two weeks in the French. That would be an awesome problem to have.''

Venus won the singles title at the Foro Italico in 1999, and is playing this clay-court event for the 12th time.

''I imagine that on the clay I have a little bit more advantage, just having played more matches on clay,'' 35-year-old Venus said after facing fellow American Vandeweghe for the first time. ''That helped a lot.''

Top-ranked Serena has a first-round bye. A three-time Rome champion, she'll open against 51st-ranked Anna-Lena Friedsam of Germany on Tuesday.

---

Andrew Dampf on Twitter: www.twitter.com/asdampf