Plenty of challengers for Peirsol in his bid to defend 100 backstroke title

There was a U.S. male backstroker who broke the 53-second barrier Monday morning at the National Aquatics Center, but his name wasn't Aaron Peirsol.

Peirsol, the world-record holder at 52.89 seconds, saw several challengers creep closer to his global standard in the semifinals at "The Water Cube", one of which was American teammate Matt Grevers.

After producing an Olympic record 53.41 in the prelims -- taking down Peirsol's mark of 53.45 from Athens in 2004 -- Grevers saw Russia's Arkady Vyatchanin and Australia's Hayden Stoeckel both produce faster times in back-to-back heats, with the 6-foot-8 standout sandwiched in between.

Vyatchanin clocked 53.06 to win his heat, then Stoeckel responded with a 52.97 in the second semifinal, with Grevers qualifying third in a lifetime-best 52.99, lowering his mark from the U.S. Olympic trials by two-tenths.

Aschwin Wildeboer of Spain qualified fourth in 53.51 and Peirsol was fifth in 53.56.

Peirsol, who lowered his world record at the U.S. Olympic trials, is attempting to become the first repeat Olympic gold medalist in the event since Germany's Roland Matthes in 1968 and 1972. Americans have captured the past three gold medals in the event, with Lenny Krayzelburg winning in 2000 and Jeff Rouse in 1996.

Warren Paoa Keahola is the only American to win back-to-back gold medals in the 100 backstroke in 1920 and 1924.