Tiger Woods endured his worst finish as a professional as he ended the WGC-Bridgestone Invitational 18 over, but he retained the world number one spot. Woods shot a final-round 77 in Ohio to finish joint 78th in the 80-man field at the Firestone Country Club. Woods' five-year reign as world number one would have come to an end if Phil Mickelson placed fourth or better. But Masters champion Mickelson carded 78 on Sunday to finish in a tie for 46th behind winner Hunter Mahan. Woods's final-round slump ended a dismal week which saw him card 74, 72, 75 for the first three rounds as he posted his highest 72-hole score in relation to par. But Mickelson failed to take advantage ending on three over for the tournament with rounds of 66, 68, 71 and 78. Despite missing out on Woods's number one tag, Mickelson remained confident that he would take his compatriot's title soon. "I hit some wild shots and the round seemed to get away from me," admitted Mickelson. "If I keep playing, it [becoming world number one] will eventually happen but I've got to get in a better rhythm. I've not played a lot and it showed today." The Firestone Country Club had previously been a happy hunting ground for Woods, who has won seven out of his 10 tournaments played there. Yet this year his combined total of 298 was 39 shots higher than the record he set for the course 10 years ago. Only Sweden's Henrik Stenson kept Woods from finishing in last place. "Shooting 18 over par is not fun," Woods said. "I don't see how it can be fun shooting 18 over, especially since my handicap is meant to be zero. It's tough. "I need to hit the ball better, I need to chip better, I need to putt better. It has been a long year." And Woods told reporters that if he did not improve his form he would not play in October's Ryder Cup. "Not playing like this. Definitely not playing like this," he conceded. "I wouldn't help the team if I'm playing like this. No-one would help the team if they're shooting 18 over par. But I think I can turn it around, we have lots of time between now and then." As it stands, Woods sits 10th in the US Ryder Cup list, meaning unless he can perform well enough at the USPGA Championship to rise into the top eight he will miss out on automatic Ryder Cup selection. He would then have to rely on US captain Corey Pavin's additional four selections to be given a place. The USPGA Championship begins on Thursday in Wisconsin. Credit: BBC