STANFORD, Calif.—Agnieszka Radwanska says that Serena Williams is a heavy favorite to win the U.S. Open, which would give her a calendar-year Grand Slam.

Radwanska, speaking to press in Stanford, said that the 21-time Grand Slam champion has few challengers and has won four majors in a row. Williams withdrew fom Stanford with an elbow injury.

“She is dominating right now and already it’s hard to stop her,” Radwanska said. “If she wants to win, she is too good. This year she is wining all the Grand Slams this year. She really wants to do it, she is on fire, especially at the US Open. There is a lot of pressure.”

Currently ranked No. 7, Radwanska has faced Serena eight times, losing each match. The Pole, who reached No. 2 back in 2013 and who has won 14 titles, said that on big occasions, Williams rarely backs off.  “She is the best tennis person,” Radwanska said. “She isn’t up and down, she remains at the same levels which is why she pretty much winning everything."

Radwanska, who lost to Serena in three sets in the 2013 Wimbledon final, said it is possible to upset Williams on occasion, but only if you play perfectly.

“She is so focused in the Grand Slams, and she doesn't want to lose a match,” she said. "There is a chance to beat her, but you have to play your best tennis and find a way. Its tough, she doesn’t let you play.”