Serena Williams: 'I'm getting better'

Serena Williams has warned opponents that she is getting better after she sealed a place in the semi-finals of the US Open.

The American fourth seed saw off Ana Ivanovic in straight sets on Wednesday as she looks to win her fourth title at Flushing Meadows.

"I feel like I'm going to get more focused and serious and start playing Serena tennis in the next couple of rounds, if I get to play two rounds," she said. "That's my goal.

"I didn't think I came into this tournament playing my best, but I definitely played better in the last two matches."

Williams will meet Sara Errani for a place in Saturday's final.