Michael Schumacher underwent a second surgery after a brain scan showed small, "surprising" signs of improvement, but grim doctors said Tuesday that they could offer no insight into the prognosis for the former Formula One champion.

Schumacher, who turns 45 on Friday, suffered critical head injuries when he fell and struck a rock Sunday while skiing in the French Alps. His manager confirmed the accident cracked Schumacher's helmet, which doctors credited for giving him a chance at survival.

Schumacher's condition stabilized somewhat after the second surgery, but he remains in a medically induced coma -- and doctors gave no prediction on how long that would last.

"We cannot tell you any more about the future," said Dr. Gerard Saillant, a surgeon and friend of the family who is in Grenoble. Saillant said it would be "stupid" to make any predictions about Schumacher's recovery.

Schumacher's body temperature was lowered to between 93.2 and 95 degrees to reduce swelling in the brain.

Tennis

Serena Williams began her season with a 6-4, 6-4 win over Andrea Petkovic at the Brisbane (Australia) International. Williams made 36 unforced errors but hit 35 winners and fired 11 aces, including one at 122 mph. Maria Sharapova received a walkover into the quarterfinals when Ashleigh Barty withdrew with a thigh injury.

  • Rafael Nadal and Andy Murray opened their seasons with victories at the Qatar Open in Doha. Nadal beat Lukas Rosol 6-2, 7-6 (7), and Murray needed just 37 minutes to eliminate wild card Mousa Shanan Zayed 6-0, 6-0.

    Baseball

    Free-agent reliever Jesse Crain agreed to a one-year deal with Houston. Financial terms weren't released. Crain made the A.L. All-Star team last season with a 0.74 ERA in 37 appearances for the Chicago White Sox. He was traded to Tampa Bay on July 29 but didn't appear in a game for the Rays because of a shoulder strain. Crain, a University of Houston alum who lives in the area, said he'll start throwing in two weeks.

    Hockey

    Connor McDavid and Curtis Lazar scored in the third period and Canada beat the United States 3-2 to win Group A in the world junior tournament in Malmo, Sweden. Canada (3-0-1) will face Switzerland in the quarterfinals Thursday. The U.S. (3-1) will play Russia.

    Track and field

    Wilbur "Moose" Thompson, the shot put winner at the 1948 Olympics, died. He was 92. Thompson had a career-best mark of 56 feet, 2 inches in London, breaking the Olympic record by more than 3 feet. He attended high school in Modesto and Modesto Junior College and USC.