San Francisco @ Colorado preview

Last Meeting ( Jun 5, 2011 ) Colorado 1, San Francisco 2

THE STORY: The San Francisco Giants are doing their best to try and chase down a playoff spot as they enter Thursday's four-game series opener against the Colorado Rockies with a four-game winning streak. With Wednesday's triumph over San Diego, the Giants pulled within 7 1/2 games of first-place Arizona in the National League West. San Francisco, which has 13 games remaining, also trails Atlanta by 6 1/2 in the wild-card race. Colorado has won three of its last four contests.

TV: 8:40 p.m. ET, CSN Bay Area (San Francisco), Root Sports (Colorado)

PITCHING MATCHUP: Giants RH Ryan Vogelsong (10-7, 2.66 ERA) vs. Rockies Jhoulys Chacin (11-11, 3.64 ERA).

Vogelsong's losing streak reached five games Saturday when he allowed three runs over eight innings against the Los Angeles Dodgers. The 33-year-old has yielded three earned runs or less in six straight outings and all but two of his 25 starts. Vogelsong won both of his previous two starts against Colorado this season, allowing one run in 14 1/3 innings.

Chacin is coming off a loss to Cincinnati on Friday in which he surrendered three runs over 5 1/3 innings. Thursday's game will be Chacin's 17th start of the year at home, where he is 6-5 with a 3.54 ERA. The 23-year-old has a 3-1 record in five career meetings with San Francisco.

ABOUT THE ROCKIES (70-78): First baseman Todd Helton and shortstop Troy Tulowitzki both sat out Wednesday's victory at Milwaukee after leaving Tuesday's contest with back and hip injuries, respectively. Catcher Eliezer Alfonzo received a 100-game suspension on Wednesday, becoming the first player to be banned twice for using performance-enhancing drugs. The 32-year-old was issued a 50-game suspension in 2008 while with San Francisco. Alfonzo batted .267 (20-for-75) with one homer and nine RBIs in 25 games with the Rockies this year.

ABOUT THE GIANTS (79-70): San Francisco has provided Vogelsong with a total of three runs of support during the righthander's losing streak. Aubrey Huff is 2-for-11 lifetime against Chacin, with both hits being home runs. Catcher Buster Posey is on his way to Arizona to continue his rehab. Posey, who has been out since May 25 with a broken leg, is expected to be hitting off a tee by next week and will gradually move on to other baseball drills.

FINAL PITCH: San Francisco's Carlos Beltran hit a pair of solo shots on Wednesday to become the ninth switch-hitter to reach the 300-homer plateau. Beltran has been tremendous against NL West rivals since being acquired from the New York Mets on July 28, going 28-for-65 (.431) with five homers and 12 RBIs.