UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

FORM 10-Q

For the Period Ended JuneSeptember 30, 2013

OR

Commission File Number 000-51274

THE FRONTIER FUND

FRONTIER DIVERSIFIED SERIES; FRONTIER MASTERS SERIES;

FRONTIER LONG/SHORT COMMODITY SERIES; BALANCED SERIES;

FRONTIER SELECT SERIES (FORMERLY TIVERTON/GRAHAM/TRANSTREND SERIES);

WINTON SERIES; FRONTIER HERITAGE SERIES (FORMERLY WINTON/GRAHAM SERIES)

(Exact Name of Registrant as specified in its Charter)

c/o Equinox Fund Management, LLC

1775 Sherman Street, Suite 2500

Denver, Colorado 80203

(Address of Principal Executive Offices)

(303) 837-0600

(Registrant’s Telephone Number)

Securities to be registered pursuant to Section 12(b) of the Act: None

Securities registered pursuant to Section 12(g) of the Act:

Frontier Diversified Series Class 1, Class 2 and Class 3 Units;

Frontier Masters Series Class 1, Class 2 and Class 3 Units;

Frontier Long/Short Commodity Series Class 1, Class 2, Class 3, Class 1a, Class 2a and Class 3a Units;

Balanced Series Class 1, Class 2, Class 3, Class 1a, Class 2a and Class 3a Units;

Frontier Select Series (formerly Tiverton/Graham/Transtrend Series) Class 1, Class 2 and Class 3 Units;

Winton Series Class 1, Class 2 and Class 3 Units;

Frontier Heritage Series (formerly Winton/Graham Series) Class 1, Class 2 and Class 3 Units

Indicate by check mark whether the registrant: (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files)    Yes  x    No  ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

Indicate by check mark whether the registrant is a shell company (as defined in Rule 1 2b-2 of the Exchange Act).    Yes  ¨    No  x

TABLE OF CONTENTS

Special Note About Forward-Looking Statements

THIS REPORT CONTAINS STATEMENTS WHICH CONSTITUTE FORWARD-LOOKING STATEMENTS WITHIN THE MEANING OF THE PRIVATE SECURITIES LITIGATION REFORM ACT OF 1995 AND OTHER FEDERAL SECURITIES LAWS. THESE FORWARD-LOOKING STATEMENTS REFLECT THE MANAGING OWNER’S CURRENT EXPECTATIONS ABOUT THE FUTURE RESULTS, PERFORMANCE, PROSPECTS AND OPPORTUNITIES OF THE TRUST. THE MANAGING OWNER HAS TRIED TO IDENTIFY THESE FORWARD-LOOKING STATEMENTS BY USING WORDS SUCH AS “MAY,” “WILL,” “EXPECT,” “ANTICIPATE,” “BELIEVE,” “INTEND,” “SHOULD,” “ESTIMATE” OR THE NEGATIVE OF THOSE TERMS OR SIMILAR EXPRESSIONS. THESE FORWARD-LOOKING STATEMENTS ARE BASED ON INFORMATION CURRENTLY AVAILABLE TO THE MANAGING OWNER AND ARE SUBJECT TO A NUMBER OF RISKS, UNCERTAINTIES AND OTHER FACTORS, BOTH KNOWN, SUCH AS THOSE DESCRIBED IN THE “RISK FACTORS” SECTION UNDER ITEM 1A AND ELSEWHERE IN THIS REPORT, AND UNKNOWN, THAT COULD CAUSE THE TRUST’S ACTUAL RESULTS, PERFORMANCE, PROSPECTS OR OPPORTUNITIES TO DIFFER MATERIALLY FROM THOSE EXPRESSED IN, OR IMPLIED BY, THESE FORWARD-LOOKING STATEMENTS.

YOU SHOULD NOT PLACE UNDUE RELIANCE ON ANY FORWARD-LOOKING STATEMENTS. EXCEPT AS EXPRESSLY REQUIRED BY THE FEDERAL SECURITIES LAWS, THE MANAGING OWNER UNDERTAKES NO OBLIGATION TO PUBLICLY UPDATE OR REVISE ANY FORWARD-LOOKING STATEMENTS OR THE RISKS, UNCERTAINTIES OR OTHER FACTORS DESCRIBED HEREIN, AS A RESULT OF NEW INFORMATION, FUTURE EVENTS OR CHANGED CIRCUMSTANCES OR FOR ANY OTHER REASON AFTER THE DATE OF THIS REPORT.

UNLESS EXPRESSLY STATED OTHERWISE, ALL INFORMATION IN THIS REPORT IS AS OF JUNESEPTEMBER 30, 2013, AND THE MANAGING OWNER UNDERTAKES NO OBLIGATION TO UPDATE THIS INFORMATION.

PART I. FINANCIAL INFORMATION

The Series of the Frontier Fund

Statements of Financial Condition

JuneSeptember 30, 2013 and December 31, 2012

The accompanying notes are an integral part of these financial statements.

The Series of the Frontier Fund

Statements of Financial Condition

JuneSeptember 30, 2013 and December 31, 2012

The accompanying notes are an integral part of these financial statements.

The Series of the Frontier Fund

Statements of Financial Condition

JuneSeptember 30, 2013 and December 31, 2012

The accompanying notes are an integral part of these financial statements.

The Series of the Frontier Fund

Condensed Schedules of Investments (Unaudited)

JuneSeptember 30, 2013