ROME — Top-ranked Serena Williams squandered a match point and lost to Jelena Jankovic 4-6, 6-3, 7-6 (5) today in the semifinals of the Italian Open.

In her first tournament back from a three-month knee injury, Williams served for the match twice in the third set and was broken both times. She hit a backhand long on match point at 5-4.

Williams took a 5-2 lead in the tiebreaker, but hit a return long and missed four consecutive shots to send Jankovic into her second final of the year.

In Saturday's final, the seventh-ranked Jankovic will meet Maria Jose Martinez Sanchez of Spain, who beat Ana Ivanovic 6-4, 6-2. Jankovic is aiming for her third title at the Foro Italico after winning in 2007 and 2008.

With Williams up 5-3, Jankovic served an ace but Williams had her hand up and the chair umpire ruled that the American had called time and Jankovic had to serve again. That drew an argument from the Serb and whistles from the crowd in the Foro Italico's new 10,500-seat stadium, which was only about half full.

"I wasn't playing the points right. I feel like I should have won, could have won," Williams said. "I think I had an apple in my throat, but it is what it is."

Sidelined since winning the Australian Open in January, Williams didn't want to dwell on the loss.

"I can't beat myself up over it — I just came back," she said.

Jankovic thought her ace should have counted in the tiebreaker.

"For me, when the server comes to the line the receiver had to be ready. That's the rule, right?" Jankovic said. "I just had to regroup and refocus. I didn't want to waste my energy — and that's what I basically did. Overall, it was good for me to be able to stay strong mentally."

Ivanovic, whose ranking has slipped to No. 58, struggled against Martinez Sanchez's serve-and-volley game. The 26th-ranked Spaniard also used an effective drop shot.

Jankovic routed Venus Williams 6-0, 6-1 in the quarterfinals on Thursday and became only the seventh player to beat both Williams in the same tournament — joining Aranxta Sanchez Vicario, Steffi Graf, Martina Hingis, Kim Clijsters, Lindsay Davenport and Justine Henin. Clijsters is the only player to achieve the feat twice.

"To beat both Williams sisters in two days is really amazing," Jankovic said. "I'm really proud of myself. It doesn't happy too often."

Jankovic and Serena, the 2002 Rome champion, had a tough time finding their groove at the start, with Williams the first to hold serve to go up 3-2.

Williams then stepped up her attacking game, trying to keep the points short with approach shots to the corners to set up easy putaways at the net.

Williams won the first set with a big forehand return of serve that Jankovic shanked way off court. However, Williams began to struggle on the longer points and Jankovic broke at love to go up 4-3 in the second set when Williams committed unforced errors in succession. Williams double-faulted to concede the second set.

Williams finished the match with 48 unforced errors to Jankovic's 35.